And irony of ironies, it may be none other than St. John of Tucson who empowered them to do so.

The trump card conservatives may hold is a constitutional rule that revenue-related bills must originate in the House. The Senate immigration measure requires that illegal immigrants pay back taxes before becoming citizens, opening the door to a House protest, dubbed a “blue slip” for the color of its paper.

House Republicans used the same back-taxes mandate for a blue-slip threat that derailed last year’s immigration conference. The new Senate bill still must survive two more weeks of voter scrutiny and contentious amendments, but several conservatives already are lying in wait for the Senate to “make the same mistake twice,” as one House GOP aide put it.

“If we get an opportunity to do it, believe me, we’ll do it,” the aide said. “I think it’s going to be a matter of who will get there first. A number of people in the House are dying to be fingered as the person who killed [the Senate bill].”…

The back-taxes provision that could trigger the blue slip came from Sen. John McCain (R-Ariz.), who continues to take heavy fire on the presidential hustings for supporting the immigration deal. McCain introduced a back-taxes amendment after a conference call in which Republican bloggers mentioned reports that the Bush administration had asked that this year’s bill not force the very costly process of tax collection among illegal immigrants.

Remember the kerfuffle last week when news broke that Chertoff, not the Democrats, insisted on pulling the back-taxes obligation from the bill? I guess St. John figured putting it back in would help soothe the savage base as it hisses at the mention of his very name. (Pssst — it won’t.) Didn’t he realize he was handing the House a blue slip by doing that? And did Bush and Chertoff realize that they were taking the blue slip away by pulling the provision? The reason they gave for stripping it from the bill was because it would have been too hard to calculate back taxes for illegals. That wasn’t a credible excuse to begin with. It’s less credible now.

It takes a majority vote of the House to actually use the blue slip for nuking purposes so we’ll need help, appropriately enough, from the Blue Dogs to pull it off. Exit question one: Given that it was right-wing bloggers who cornered McCain about the back-taxes provision, how much thanks do we owe them? Exit question two: If the blue slip is duly served, won’t the Senate just strip the back-taxes provision and send it back?

Update: Well, whoopee-dee-do!