UPDATE 4 p.m.: Rubio spokesman Alex Conant says Republican Leader Mitch McConnell will soon offer an alternative proposal that Rubio will support. "We're also looking at other alternatives for extending it without raising taxes on job creators."

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Republican Sen. Marco Rubio, who expressed openness before to an extension of the payroll tax cut, now opposes the idea.

The reason: The Democrats' proposal, announced Monday, would pay for the break with a permanent 3.25 percent tax surcharge on gross income over $1 million.

"The senator would oppose a payroll tax cut extension that’s paired with tax increases on job creators," spokesman Alex Conant said.

Rubio was favorable to the idea when first proposed by President Barack Obama. But then came the task of paying for it, and Republicans have held firm against any tax increases.

The plan is expected to come up later this week and likely fail, giving Democrats an avenue to attack the GOP as cruel to average workers. Democrats will also try to paint rivals as supporting a tax increase, since the current 2 percent payroll tax cut expires Dec. 31. Republicans will say it will hurt jobs.