President Obama said Thursday that Donald Trump's idea for a "deportation force" to help expel immigrants in the U.S. illegally was not only impractical, but contrary to the character of the country.

"The notion that we’re going to deport 11, 12 million people from this country -- first of all, I have no idea where Mr. Trump thinks the money’s going to come from," Obama said in an interview with ABC News on Thursday, estimating it would cost billions of dollars.

But beyond that he warned such a program -- perhaps a revival of a controversial Eisenhower-era effort -- would diminish America's standing abroad.

"Imagine the images on the screen flashed around the world as we were dragging parents away from their children, and putting them in what, detention centers, and then systematically sending them out," Obama said. "Nobody thinks that that is realistic. But more importantly, that's not who we are as Americans."

The president has been eager to engage in the 2016 presidential contest, particularly on the issue of immigration. At a recent Congressional Hispanic Caucus Institute dinner he noted how far the GOP had come on the issue since his predecessor, George W. Bush, had warned against "exploiting the issue of immigration for political gain" as he tried but ultimately failed to pass major immigration reform.

Obama said in the ABC interview that there has always been an anti-immigrant strain in America, "ironically, from folks who themselves two generations back or even one generation back were immigrants," he noted.

"It's the job of leaders not to play into that sentiment," he said. "We don't want, I think, a president or any person in a position of leadership to play on those kinds of fears."

It’s game on between Marco Rubio and Ted Cruz over immigration policy, as the two Republican presidential candidates try to gain an advantage on an issue that is fast becoming a defining one in the primary campaign.

Rubio declined to enter the fray over immigration at this week's debate. His silence led to renewed scrutiny of his former advocacy for a bill that created a path to citizenship for some 11 million immigrants here illegally.

But on Thursday, Rubio went on offense, seeking to regain footing among conservative voters who prefer a harder line.

Rubio fired at Cruz, the Texas senator who has been his chief critic on immigration, noting that Cruz too had once favored giving immigrants legal status and expanding the number of work visas.

“If you look at it, I don’t think our positions are dramatically different,� Rubio said during a campaign stop in South Carolina. “I do believe that we have to deal with immigration reform in a serious way, and it begins by proving to people that illegal immigration is under control.�

Cruz did offer amendments during to the Rubio-backed immigration overhaul bill in 2013, including one that would have stripped the potential for citizenship for these immigrants, limiting them to a lesser but still-legal status. But the amendment was viewed as a spoiler that would derail the broader bipartisan bill, and it was not accepted.

Cruz this week refused to cede any ground on the issue as he seeks to dominate the party’s right flank.

Cruz said during the debate that he was getting “angrier and angrier� as former Florida Gov. Jeb Bush and Ohio Gov. John Kasich pushed back against Donald Trump’s calls for deporting all immigrants living illegally in the U.S.

“We saw one Republican after another giving an ode to amnesty,� Cruz said Wednesday on Fox. “One after another saying we have to be compassionate…. There is nothing compassionate about a bunch of politicians saying, 'I’m so compassionate, I’m going to give away your job.'�

For immigration advocates, the difference between changes that would provide legal status or citizenship are key. They have long argued against legality alone, calling it second-class.

Both Rubio and Cruz are the children of Cuban immigrants, but with very different family experiences.

Rubio’s parents both came to the U.S. from Cuba, his father a bartender and mother a maid and homemaker, and he speaks fluent Spanish. Cruz’s mother is a third-generation Irish-Italian American whose second husband was a Cuban immigrant; and they were both college-educated computer programmers.

Dr. Ben Carson's foray into the foreign policy arena earned a polite rebuke from some of President Obama's top foreign policy advisors.

In Tuesday's Republican debate the presidential hopeful warned that not only was Russian President Vladimir Putin trying to "spread his influence throughout the Middle East," but the Chinese are engaged there as well.

"We also must recognize that it's a very complex place. You know, the Chinese are there, as well as the Russians, and you have all kinds of factions there," he said.

Asked about the comment Thursday at a White House news conference, National Security Adviser Susan Rice first seemed puzzled by the remark, asking what kind of involvement he thought China might have had -- diplomatic or military.

"I have not seen evidence of Chinese military involvement in Syria," she said.

Ben Rhodes, Rice's top deputy, was more willing to plunge into the political debate.

"I think it's worth stepping back and noting that China makes it a practice to not get extended into military conflicts in the Middle East," he said. "In many respects, their policy over many years, if not decades, has been to not be overextended in military exercises."

Praising the speed and quality of the U.S. Postal Service may not be the most fashionable thing to do these days – but Bernie Sanders has never worried about what is fashionable.

His years of standing by embattled postal workers paid off on Thursday, when he won the endorsement of their 200,000-member union. The union enthusiastically lined up in support of Sanders, who delivered a closed-door speech to its members in Las Vegas last month warning about the postal service being under attack from billionaires looking to privatize it.

“We should judge candidates not by their political party, not by what they say, not by what we think they stand for, but by what they do,� American Postal Workers Union President Mark Dimondstein said in a statement. “Bernie Sanders is a fierce advocate of postal reform. He staunchly opposes postal privatization, and supports enhanced postal services, including postal banking.�

The endorsement gives Sanders a much-needed boost among organized labor. Despite his record as a die-hard advocate for unions over decades in Congress, the big unions have been lining up in support of Hillary Rodham Clinton. Union leaders are more confident of her ability to win the general election.

Two new polls out Thursday morning show Maryland Gov. Martin O’Malley with a glimmer of daylight among Democrats nationally and Texas Sen. Ted Cruz fighting it out in his home state primary with Donald Trump.

Let’s get to the CBS News/NYT Democratic poll first. The national survey taken Nov. 6-10 shows Hillary Rodham Clinton, the former secretary of State, with support from 52% of Democrats, compared with 33% for Vermont Sen. Bernie Sanders, and 5% for O’Malley. Clinton’s numbers are in line with other recent national polls and a slight uptick from the previous CBS/NYT poll taken more than a month ago, which had her at 46% support among Democrats.

O’Malley, however, was below 1% in the prior poll, suggesting that he may be getting a small bump from his exposure in last month’s debate and the formal declaration from Vice President Joe Biden that he will not run. His 5% support is hardly a mandate, but it might help him get more consideration during this weekend’s Democratic debate in Iowa.

Now onto the University of Texas/Texas Tribune poll of the nation’s second most populous state. Cruz is counting on Texans to make him a contender for the nomination and hoping that Trump fades by the time voters there hit the polls on March 1. The poll shows both Cruz and Trump with 27% support. Retired neurosurgeon Ben Carson is showing in third place (13%), and Florida Sen. Marco Rubio comes in fourth (9%).

That’s a change from an October poll of Texans taken by KTVT, which showed Trump (22%) and Carson (23%) slugging it out, with Cruz (14%) fighting former Florida Gov. Jeb Bush (13%) for third place.

Clinton leads Texas Democrats handily, with 61% of the votes, compared with 30% for Sanders.

Donald Trump picked a new Twitter target Thursday morning after the Wall Street Journal editorial board published a piece criticizing his debate remarks on tariffs for Chinese goods. Trump called the Chinese “currency manipulators� in Tuesday’s GOP debate and America’s “No. 1 abuser� in international trade.

The Journal disagreed with Trump’s take.

“The businessman thinks economic mercantilism is a political winner, but we doubt that starting a trade war that raises prices for Americans would turn out to be popular,� the WSJ editorial board wrote.

The Journal went on to say Trump proposes protectionist policies reflective of former President Herbert Hoover.

Trump called for a correction in the Journal and said he wants smart trade deals. He said the TPP will allow China to manipulate the market in future — the country is already doing it.

“Economists estimate that the yuan is undervalued anywhere from 15% to 40%,� Trump said in a Nov. 9 WSJ article. “Through manipulation of the yuan, the Chinese government has been able to tip the trade balance in their direction by imposing a de facto tariff on all imported goods.�

Quartz later debunked Trump’s claim on the yuan’s undervaluation.

Protests at the University of Missouri and Yale University sparked a response from Republican presidential candidate Ben Carson on Wednesday. Carson called the student activists' behavior “infantile.�

“This is just raw emotion and people being manipulated…by outside forces who wish to create disturbances,� he told Megyn Kelly during an interview on Fox News. He didn't clarify what "outside forces" he was talking about.

Alleged racial insensitivity on both campuses led students to protest through hunger strikes and to lash out at members of the media covering the demonstrations. Members of the Missouri football team also refused to play until University of Missouri system President Tim Wolfe resigned; he did so on Monday.

At Yale, Carson’s alma mater, students protested Halloween costumes they saw as offensive and the reaction of an administrator's wife, who urged them to discuss the costumes.

Carson said the violent pushing of journalists on Missouri’s campus shows a “dangerous trend.�

“When we get to a point where a majority can say ‘I don’t like what you’re doing, that’s offensive, and therefore I have a right to be violent toward you or deprive you of rights because I don’t like what you’re doing,’ that really goes against the grain of our constitutional rights,� he said.

Too much tolerance for incidents like these can end up violating others’ civil rights, he said.

"The officials at these places must recognize that and have the moral courage to stand up to it,� he said. “Because if they don't, it will grow, it will exacerbate the situation and we will move much further toward anarchy than anybody can imagine, and much more quickly."

Ben Carson: We're being too tolerant of infantile behavior

In the days leading up to Tuesday night’s Republican debate, moderators Neil Cavuto and Maria Bartiromo, anchors of Fox Business Network — on which it aired — and Gerard Baker, editor in chief of the Wall Street Journal, made it clear they would not just be hosting a discussion of economic policy.

No, perhaps even more important, they would be restoring order to what had become a media-fomented Fight Club in which debate moderators seemed intent on making a bigger splash than the candidates. In contrast, Cavuto swore that he and his colleagues would be "invisible."

"Unobjectionable" would be closer to the mark. The medium continued to draw as much attention as the message, just in a more candidate-considerate way.