Extending the tax rates, of course, garners near-universal acclaim. But Republicans view other parts of the deal, such as the temporary payroll-tax deduction, the $56 billion toward jobless benefits, and myriad Obama-favored tax credits, as hold-your-nose compromises.

For an example of the grumbling, see Sen. John McCain of Arizona, who wrote in a Twitter message Friday that he has "serious concerns." Nevertheless, he will support the agreement because "Americans can't afford tax increases."

While McCain's grudging acceptance appears to reflect the caucus consensus, the whip count is far from certain. Indeed, with the Senate ready to vote Monday on whether to take up the tax agreement, Republicans, especially those who are Tea Party darlings, face a pressing decision: Will they fight?

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