Tomorrow the Senate is set to vote on whether to end debate on the big transportation and housing bill that has been working its way through the Senate. The vote is a key test as to whether Mitch McConnell is truly losing control of his caucus as a bloc of Republican Senators indicates a willingness to break with the GOP leadership and join with Democrats in governing.

Democratic aides say that they’ve received reports from GOP Senators that McConnell’s operation is whipping for No votes today, and Roll Call notes that McConnell announced on the Senate floor this morning that he was “encouraging opposition to the bill.” This is key, since Dems are viewing this bill as an important test of whether their efforts to drive a schism into the GOP caucus by wooing compromise-minded Republicans is working. The question is whether enough Republicans can be induced to support the bill, given that infrastructure spending has historically gotten bipartisan support.

As Democratic Senator Debbie Stabenow put it: “The reality is that Republicans and Democrats both drive on roads. Republicans and Democrats care about whether or not the bridge they’re driving over is safe, and this should not be a partisan issue.”

Six Republican Senators voted for the bill in committee, but conservatives — and McConnell — continue to oppose the bill because its spending levels are higher than those on the House version of the bill. Susan Collins continues to advocate strenuously on its behalf, challenging Tea Party claims about its spending levels, and insisting that Republicans should pass the bill to allow the two chambers to proceed into conference negotiations over the bill.

“I think it’s unfortunate that he’s not allowing regular order to take place to finish the THUD bill and going to conference with the House,” Collins said. Republicans have increasingly been unwilling to proceed to conference on the budget, too — after all, that would require them to enter into negotiations designed to bring about a compromise.

Democrats are actively using the confrontation to try to put McConnell in a difficult spot. Rand Paul offered an amendment today (that was defeated) that would have nixed aid to Egypt. It was supported by a bevy of Tea Party Senators, and McConnell voted No on tabling the motion. I’m told Dems allowed the vote explicitly to put McConnell in the position of having to decide whether to break with Tea Partyers in his caucus.

The big vote — on whether to end debate on the measure — is set for this afternoon, and if Dems can persuade a handful of Republicans to again vote to move the bill forward on the floor, it will be a sign that a sizable bloc of Republicans in the Senate remains willing (as on executive nominations) to break with the leadership. That could bode well for this fall’s coming confrontations over the debt limit and replacing the sequester, since Dems still hope to win over a handful of Senate Republicans in a manner that will ratchet up pressure on House Republicans to cave on one or both fronts.

That said, Dems already see cause for pessimism in the mere fact that McConnell is lobbying GOP Senators to oppose the transportation bill. That’s because McConnell’s apparent need to prove he can kill this bill — in the face of whatever rebellion we’re seeing among Republicans inclined to support it — suggests he needs to reestablish that he can indeed control his caucus and prevent any compromising with Dems from moving forward, for his own political purposes. That doesn’t bode particularly well for what’s to come.

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UPDATE: The big cloture vote on the transportation bill is tomorrow. I’ve edited the above to correct.

UPDATE II: McConnell actually voted No on a motion to table the Egypt aid motion; he voted with the Tea Partyers. I’ve edited the above to correct.

Greg Sargent writes The Plum Line blog, a reported opinion blog with a liberal slant -- what you might call “opinionated reporting” from the left.