Here’s something to keep an eye on in order to gauge whether Mitch McConnell’s escape-hatch plan on the debt ceiling — perhaps the last way out of this impasse — can pass the House of Representatives over conservative opposition.

Late yesterday GOP Rep. Joe Walsh, who is backed by the Tea Party, began circulating a letter among GOP colleagues that urges GOP leaders John Boehner and Eric Cantor to publicly oppose the McConnell plan and even to oppose it coming to the floor for a vote.

A Senate Republican aide tells me that GOP aides will be closely watching the number of signatures it amasses in order to gauge whether the McConnell proposal can get through the House.

“On behalf of the millions of Americans we represent, we strongly urge you to both publicly oppose Senator McConnell’s plan to raise the debt ceiling and ensure it never comes to the House floor for a vote,” the letter reads.

“We understand Senator Mitch McConnell’s frustration with the debt ceiling debate, but his plan enables Congress to avoid making the hard decisions,” the letter continues. “This plan is nothing more than politics as usual: it passes the buck to President Obama and robs the American people of their voice in Congress.

Senate Republican aides are hoping that the letter doesn’t amass more than 50 signatures, since presumably most GOPers who do sign it are likely to vote No on the McConnell proposal. Walsh’s goal is to get more than 100 signatures on the letter, which would be a major statement of opposition to the McConnell plan in the House and would raise doubts about whether it can pass.

The Senate Republican aide I spoke to says those who support the McConnell scheme as a plan of last resort are holding out for what might be called a “what now” moment. Here’s how this may play out: The House passes “cut cap and balance” today, it fails in the Senate over the weekend; and by Monday everyone finally realizes that there’s no way the White House and Congressional leaders will ever reach a deal exchanging spending cuts for revenue hikes. At that point, the aide predicts, everyone may throw up their arms and say, “what now?” It’s only then that House Republicans — who got their chance to vote Yes on spending caps and a balanced budget amendment — will realize that they have no choice but to pass the McConnell proposal.

After all, a new Gallup poll finds that an astonishing 57 percent of Republicans want their leaders to reach a debt ceiling compromise, even if they disagree with it. There’s no longer any doubt about where public opinion is on this question. But in Congress, it may require a “what now” moment before rank and file Republicans embrace the inevitable.

For now, keep an eye on this letter.