Fresh off their victory Tuesday night, Republicans may be eager to pick a fight with President Barack Obama during the lame duck session. In particular, expect a tense battle over the budget for the 2015 fiscal year. But if the GOP isn’t itching for a fight already, they surely will be when Obama takes an executive action on immigration reform that could allow millions of undocumented immigrants to live in the U.S. without fear of deportation.

Immigration advocates have long lobbied the president to extend his 2012 executive action—the Deferred Action for Childhood Arrivals (DACA) initiative, which granted children of undocumented immigrants a reprieve from deportation, as long as they were under the age of 30 and brought here before 2007. DACA also grants undocumented immigrants permission to work, although there’s no guarantee the federal government won’t revoke that permission.  

The administration’s legal basis for this action rests on prosecutorial discretion. The Department of Homeland Security has limited resources and must make decisions on how to deploy them. As I explained in August, Obama must make sure that his use of prosecutorial discretion does not cross the line into lawmaking, which is the role of Congress. DACA, most experts agree, does not cross that line.

Advocates were buoyed at the end of June when Obama directed his staff to produce a list of actions on immigration that he could take by the end of the summer. The Washington Post reported that measures under consideration included an order that would grant up to five million undocumented immigrants a temporary reprieve from deportation. Although the reports suggested the Administration had not yet decided upon a course of action, Republicans were furious. Senator Jeff Sessions called it an "exceedingly grave threat that go[es] beyond anything we've ever seen in this country" and "threatens the very constitutional framework of our republic and the very ability of this nation to even have borders." Some even floated the possibility of impeachment.

Obama didn’t seem particularly fazed by the Republican reaction. Then the border crisis hit. Republicans blamed the influx of Central American kids on DACA and warned that another executive order would invite even more would-be immigrants to attempt the journey. In early September, Obama announced he was delaying the executive action until after the election. "The truth of the matter is that the politics did shift mid-summer because of that problem," he said. It was a nakedly political decision that infuriated immigration advocates.  But Obama promised immigration advocates that he had not given up on some kind of action—he had merely delayed it. By December 31, he promised, he would do something. On Tuesday, White House Press Secretary Josh Earnest reiterated the president’s commitment to taking action before New Years.

Immigration advocates aren’t so sure—and they certainly aren’t going to leave things to chance. They are planning rallies to push him to make the announcement sooner rather than later. Republicans, meanwhile, are warning that any action by Obama would kill the chances of Congress passing an immigration reform bill. Their threat probably won’t mean much, since few people think Congress is capable of passing an immigration bill at this point. But even some reporters on the left, like Vox's Matt Yglesias and Ezra Klein, are now doubtful that Obama will make a big move.

The threat of reprisal from immigration reform advocates, however, could resonate with the White House. Obama isn’t running for reelection, but other Democrats will be on the ballot in 2016 in states with large Hispanic populations and he’d like them to win. Plus there’s the small fact that Obama actually supports immigration reform—and, with Republicans now in unified control of Congress, extending DACA is one of the last big things he can do before leaving office.