The shutdown battle has become, essentially, a high-stakes blame game.

Republicans tried to raise those stakes yesterday by offering a piecemeal plan to reopen popular parts of the federal government, in the hopes that it would be politically costly for the Democrats to oppose things like aid to veterans. The bill failed to move forward.

But in a note to clients, Potomac Research Group's Greg Valliere says that President Obama still has one "trump card" of his own left to play, Social Security:

THE HEAVY ARTILLERY: We have thought for weeks that Obama would play the Social Security trump card if there was no deal on the debt ceiling by mid-October. This is one of several reasons why we think a default is unlikely, and it's one of several reasons why Boehner will capitulate; the only questions are when and under what terms. We think he may get a few crumbs, but no major concessions.

It looks increasingly likely that the debate over the government shutdown will roll into a debate over the debt ceiling.

"GOP strategists like Karl Rove surely know that it's just a matter of time before President Obama throws a game-changer — warning senior citizens that their Social Security checks won't be mailed because of John Boehner," Valliere notes.

Seniors tend to care about Social Security. Moreover, they tend to let Congress know when they're unhappy. And Congress listens, because seniors tend to vote. Valliere believes this will be the turning point in the debate.

As far as this week, "There are signs of dissent within the House GOP as moderates step up their campaign for a clean bill that reopens the government," Valliere writes. "This evolving shift in the House will set in motion the eventual capitulation by John Boehner, which could come by late next week."