As a bipartisan group of senators prepares to unveil a wide-ranging immigration reform bill, a key Republican member of that group may be putting the brakes on his own legislation. 

Sen. Marco Rubio (R-Fla.) has been one of the most vocal lawmakers from either party to push for immigration reform this year. The freshman senator broke with the majority of his party in January by saying he was open to bestowing legal status to undocumented immigrants, a crucial sticking point in the debate. 

However, with Congress moving swiftly on the issue, Rubio has repeatedly cautioned his colleagues to slow down.  

Last week, his office issued a statement warning that reports of an imminent immigration deal were "premature." That was "a deliberate attempt by Rubio to slow down the momentum toward a deal on immigration," say the Washington Post's Chris Cillizza and Sean Sullivan, who point out that the statement dropped the same day that Sen. Chuck Schumer (D-N.Y.) was scheduled to discuss the pending legislation on NBC's Meet the Press

Rubio has also raised concerns about the potential cost of immigration reform. And last Friday, he said the pending bill would be a "starting point, not a take-it-or-leave-it proposition," fueling fears among reform advocates that a protracted debate would give opponents sufficient time to rally and defeat the legislation.

So why is Rubio raising these concerns now?

Immigration reform is still deeply unpopular with many conservatives, and publicly calling for a slower, more deliberative process would give Rubio an easier sell to the party base once a final compromise is reached.

Here's the Post's Sullivan on that point: 

Sure, immigration reform is a political necessity, but caving in to Democratic demands and embracing new laws conservatives will grouse about is politically perilous too — especially for someone who might run for president. In other words, if Rubio is to embrace the final proposal the 'Gang of Eight' comes up with, it can really only be after extended consideration, and the perception that he regularly pushed back against the Democrats in the group. [Washington Post]

Others have similarly attributed Rubio's newfound hesitancy to his presumed presidential ambitions. 

"One word, or rather number: 2016," says Bloomberg's Francis Wilkinson. "Rubio's call to slow down the process on an issue that has been negotiated on both sides of the Capitol for the better part of a decade aligns him — momentarily — with the party's all-important conservative base." 

There's also been much speculation that Rubio is prepared to ultimately walk away from the legislation he helped to create. According to Politico, that prospect, "more than any other dynamic," has been driving the proposal's fine-tuning.

For now, Rubio is still expected to sign on to the final bill. However, spurning the legislation could be a political win-win, since it would allow him to "say that he wanted to make a deal, but the other side was too unreasonable in its demands," says the American Conservative's Daniel Larison.

As Talking Points Memo's Benjy Sarlin notes, there is some recent historical precedent for Republicans initially supporting controversial Democratic-backed bills, only to wind up voting against them. However, he argues that Rubio's actions are more likely "a wink to conservatives without any actual substantive concerns behind it." Rubio has already placed himself to the left of his party on the issue, so there's no real benefit to turning back now, Sarlin argues.

Conservative Washington Post columnist Jennifer Rubin made a similar argument Monday, writing that Rubio's communications director told her that the senator has only been angling for more leverage in the negotiations, not attempting to stall the bill into oblivion. 

"No one has more to gain than Rubio if immigration reform passes — and passes with a good share of the GOP support," says Rubin. "And, in turn, the Republican Party has much to gain by jump-starting legislation that President Obama did not champion in his first term."