As of last week, congressional Republicans, especially in the House, had a lengthy list of demands they expected to have met before extending the payroll tax break. As of today, the ransom note has been winnowed, basically to a single condition.

Regarding that legislation, Don Stewart, a spokesman for Senate Minority Leader Mitch McConnell emails me with the following statement: “The Leader will not support any bill without the Keystone XL language as part of the agreement.”

House Speaker John Boehner is also insisting that he’ll amend any Senate-passed payroll tax cut bill to add the Keystone provision to it, if it’s not already in there. So Senate Majority Leader Harry Reid and President Obama now have a choice: stick to their guns and object to the provision — at the risk of allowing the payroll tax cut (and unemployment insurance and the Medicare “doc fix”) to expire? Or give in to the GOP.

Are Republican leaders seriously taking the position that taxes should go up for 160 million Americans unless they get the Keystone XL pipeline? Yes, that’s pretty much the GOP position.

Note, as of a few days ago, both parties were pushing for measures in the payroll fight the other party found objectionable — Democrats wanted a surtax on millionaires and billionaires; Republicans wanted Keystone. Democrats, hoping to reach a deal, effectively said yesterday, “We’re willing to drop our demand.” To which Republicans responded, “Give us what we want or else.”

There appears to be a difference in the way the parties negotiate.

At this point, there are plenty of questions to consider. At the top of the list: are Republicans bluffing? I rather doubt it, since most GOP lawmakers don’t like the idea of a middle-class tax break anyway.

Another question: exactly how many jobs are we talking about? The standard Republican/Fox News talking point is that the Keystone XL project would create 20,000 jobs. Nonpartisan estimates suggest that figure is wildly inflated, and one independent report concluded that “the project could actually destroy more jobs than it creates.”

There’s also the inconvenient details as to how the pipeline decision would be made. The Republican demand isn’t exactly that the project get a green light, but rather, that the decision be expedited. The issue is in the hands of the State Department, which has warned Congress that forcing the issue may very well kill the entire initiative. Indeed, Reuters reported last week that a deadline imposed by Congress could “effectively rule out a permit for the project.”

Republicans don’t seem to care, and seem to think they can eventually get their way with the administration, but it’s an angle worth watching. If Dems cave on this point in the payroll fight, the project may die anyway.

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.