Senate Democratic leaders are circulating a letter to President Obama, pledging full support for his coming executive action to shield millions from deportation. The letter makes the case that bringing all these people out of the shadows will be good for the economy, and that Obama has broad authority to act to improve the immigration system for the good of the country where he can — framing the message that Democrats hope to unify behind when this battle heats up.

That suggests top Democrats will have Obama’s back. But it also raises a question: What happens if a half dozen Senate Dems defect and side with the GOP against Obama’s executive deportation relief?

Immigration advocates are warning that this is a real possibility — one that could have a serious impact on the politics of this fight if and when a government shutdown battle looms — and they are preparing to exert maximum pressure on those Democrats they deem at risk.

“We are preparing to pressure them at home and in Washington, to let them know that there will be hell to pay if this happens,” Frank Sharry, the executive director of America’s Voice, tells me.

Among the Democrats believed to be at risk are Joe Manchin, Heidi Heitkamp, Jon Tester, Claire McCaskill, and Joe Donnelly. Angus King (who is an independent but caucuses with Dems) is also a question mark.

The problem, advocates worry, is that if these Democrats come out against any Obama executive action, it could complicate the political battle to come. Republicans are expected to try to pass legislation rolling back whatever Obama does. Democrats will try to block it. But if Republicans can get 60 votes — which they could do if enough Dems defect — the president would then have to veto it. That could make the politics of this battle worse for Obama: Not only is he acting unilaterally; he’s also facing bipartisan opposition within Congress that is requiring him to protect those unilateral actions with a veto.

Sharry says such an outcome risks tilting public opinion against the president if this then turns into a government shutdown fight. If legislation fails to overturn Obama’s actions, Republicans may then seek to use government funding fights as leverage against them, as Ted Cruz wants them to do. If some Democrats oppose Obama’s executive action, it could then become harder to pin the blame for a looming shutdown exclusively on the GOP.

“The big question is whether Republicans can claim they have bipartisan support in opposing the president,” Sharry says. “When it comes to shutdown politics, will this be seen as a straight-up partisan standoff, or will there be confusion in the public mind over who is to blame?”

“If you’re a Republican looking to kill this in the crib, you want Obama to get weakened by Democratic defections and by public opinion turning against him,” Sharry continues. “Far and away the most important factor is if the president stays strong throughout. But these Democrats will have to decide: Do they vote reliably with the president, or do they vote reliably with Ted Cruz?”

“Our hope and expectation is that Senate Democrats unify and rally behind the president, assuming he does the right thing,” adds Chris Newman, legal director for the National Day Laborer Organizing Network. “Republicans were the ones divided on immigration. If the Democrats were to divide now, that would put them at a considerable disadvantage.”

The Democratic Party in recent years has been more unified on immigration than it has been historically — as evidenced most recently by the overwhelming support of Senate Dems for the immigration reform bill in 2013. But the politics of Obama acting alone may look a bit different to some of them.  The entire progressive infrastructure will gear up to defend Obama’s executive action. But some progressives will likely be enraged by any Democratic defections, particularly if it undermines the chances of securing Obama’s executive actions in the face of Republican opposition — and a loss here could be a disaster for the Democratic Party overall.

Democrats saw what happens this year when its base doesn’t turn out,” Markos Moulitas, the founder of DailyKos, tells me. “There’d be no better way to ensure more electoral pain than to continue to run against the interests of its own supporters.” Moulitsas went after Senator Tester very, very hard after the latter voted against the DREAM Act, a preview of what might happen in this case. 

All of which is to say that there is a very difficult and treacherous political fight ahead, and that even if Obama does act to bring deportation relief to millions, securing it is anything but assured.