With momentum on immigration reform grinding to a halt, Sen. Marco Rubio (R-Fla.) is trying to motivate Republicans by invoking the ultimate conservative bogeyman: President Barack Obama.

"I believe that this president will be tempted, if nothing happens in Congress, he will be tempted to issue an executive order like he did for the DREAM Act kids a year ago, where he basically legalizes 11 million people by the sign of a pen," Rubio told Preston Scott on Tallahassee's WFLA radio. "Now, we won't get an E-Verify, we won't get any border security. But he'll legalize them."

Rubio is the Republican face of bipartisan immigration bill that passed the Senate in June, in what is increasingly looking like a disastrous political decision for the once-rising star, given the GOP base's opposition to the legislation. The bill includes liberal priorities, like a path to citizenship, as well as conservative ones that Rubio mentioned: Beefed-up border control and tougher legal status verification under a program called E-Verify.

Rubio's suggestion is that unless the GOP-led House gets on board, Obama will ram through a Democratic wish list of immigration reforms, while leaving Republicans in the lurch.

The "executive order" Rubio is referring to happened in June 2012, when Obama issued a policy directive that gave people who would have been eligible for the failed DREAM Act a temporary reprieve from the threat of deportation. (The DREAM Act would have provided a path to legal residency to certain, qualified undocumented immigrants who were brought to the U.S. as children.)

The move angered conservatives, and some liberals have wondered if he could issue a similar order for all of America's 11 million undocumented immigrants.

"It's not an empty threat," Frank Sharry, executive director of immigration advocacy group America's Voice, told Politico. "If Republicans block reform with a path to citizenship, immigration reform activists will look at all their options, including broad executive action."

The White House, however, denied that it was planning to do any such thing.

"The only solution to this problem is for Congress to fix the broken immigration system by passing comprehensive reform," said White House spokesman Bobby Whithorne.

President Obama expressed a similar sentiment in January to Telemundo's Jose Diaz-Balart:

I'm not a king. You know, my job as the head of the executive branch ultimately is to carry out the law. And, you know, when it comes to enforcement of our immigration laws, we've got some discretion. We can prioritize what we do. But we can't simply ignore the law. [NBC Latino]

Several conservatives agreed with the White House that Rubio was making empty threats.

"Rubio is trying to play on the worst fears of conservatives to push through his amnesty," Roy Beck, leader of anti-immigration group NumbersUSA, told Politico. "I don't think conservatives are going to buy it.”

Conn Carroll, conservative writer for The Washington Examiner, also found fault with Rubio's logic:

While liberals might hope Rubio's warning could convince Republican lawmakers to compromise on immigration reform, many, like The Washington Post's Greg Sargent, aren't holding their breath.

"The willingness to believe the worst of Obama collided with hatred of amnesty, and hatred of amnesty won," Sargent wrote. "Still, it was worth a try."