Remember, at the White House this afternoon, President Obama issued a pretty clear ultimatum in the fiscal cliff fight. "Revenues have to be part of the equation in turning off the sequester," he said.
That demand infuriated Republicans. They said Obama moved the goalposts. He was certainly strong-arming them -- arguably even attempting to saddle them with the blame if they rejected the demand and the deal fell apart.
McConnell's move more or less flips the script. Now if Obama and Democrats decide to insist on including the sequester in a single New Year's Eve deal, they'll be the ones objecting to an extension of middle-income tax cuts. After spending months smacking Republicans for holding those tax cuts hostage.
That's going to be hard to explain away. But if Obama and Harry Reid take McConnell up on his offer, he'll have nothing, other than his own power of persuasion, to force Republicans to accept any new revenues for anything next year. Not to turn off the sequester. Not to do further deficit reduction. Not if Republicans pick a debt limit fight.
It's a gut check moment for Obama. If he means what he says that he won't negotiate with Republicans over the debt limit, this might not be such a bad offer to accept. But if he has any doubts, then he's either got to say no, or prepare to take a beating in the next few months.