The criticism of the House Republicans this week has been fierce.

One senator accused them of “playing politics” and said their actions would “hurt American families and be detrimental to our fragile economy.”

Another said there was “no reason” for House Republicans to oppose a two-month extension of the payroll tax cut, and added that “What is playing out in Washington, D.C., this week is about political leverage, not about what’s good for the American people.”

And a third urged House Republicans to pass the short-term payroll tax cut bill because “this is best for the country, as well as for all the individuals who are affected.”

The remarkable thing about all of that criticism is that it did not come from Democrats. Rather, the angry, disbelieving rhetoric came from Senate Republicans, who until now have maintained a nearly airtight alliance with their Republican colleagues in the House.

That Republican unity during the last three years has created enormous political problems for President Obama, who has been repeatedly stymied in his efforts to push forward his agenda. Just about every time Mr. Obama sought to drive a wedge between House and Senate Republicans, he failed.

But the vote to authorize the short-term payroll tax extension — which passed the Senate by a vote of 89 to 10 — suggests that the Republican unity may be cracking in the face of a searing reality for the members: 2012 is an election year.

For the Republican senators who must face the voters next year, especially those in tough races, the idea of allowing payroll taxes to increase on Jan. 1 is not a palatable option. Their willingness to accommodate the ideological imperatives of the House Republicans may be weakening.

Scott P. Brown, the Republican senator from Massachusetts, was the one to accuse his House colleagues of “playing politics.” In comments before the House vote on Tuesday to reject the Senate compromise bill, he went even further.

“We are Americans first; now is not the time for drawing lines in the sand,” said Mr. Brown, who faces a tough re-election battle against Elizabeth Warren in a state — Massachusetts — that leans Democratic. He called the actions of the House Republicans “irresponsible and wrong.”

Dean Heller, a freshman senator from Nevada, was the one who referred to “political leverage.” Mr. Heller echoed Democratic lawmakers (and Mr. Obama) who argued that a yearlong extension of the tax cut was preferable. But Mr. Heller, who was appointed to fill the term of John Ensign and faces election next year, said he supported a short-term deal in the meantime.

And Richard G. Lugar, a veteran Republican senator from Indiana who also faces re-election this year, was the one who said the two-month deal was “best for the country.”

“I’m hopeful there are a majority of Republicans and Democrats today who will proceed,” he said on Monday on MSNBC.

The decision by senators like Mr. Brown, Mr. Heller and Mr. Lugar, to break ranks with House Republican orthodoxy may help them withstand challenges next year.

But the every-man-for-himself approach threatens to undermine the successful political strategy that Republicans have been employing against Mr. Obama since the day he took office three years ago.

In the early part of Mr. Obama’s term, Republicans voted en masse against the stimulus and health care bills, forcing the president to rely solely on Democratic votes and setting up political arguments that helped them retake the House during the 2010 midterm elections.

Since then, the Senate Republicans — under the guidance of Senator Mitch McConnell of Kentucky — have stuck by Speaker John A. Boehner through difficult negotiations over the budget, the debt ceiling, the Bush-era tax cuts and other divisive issues.

Now, Republican senators who voted for the short-term payroll tax cut are saying they recognize that the dispute, and ones like it that could arise next year, are a potential threat.

“It is harming the Republican Party. It is harming the view, if it’s possible anymore, of the American people about Congress,” Senator John McCain, Republican of Arizona, said on CNN Tuesday. “We’ve got to get this resolved and with the realization that the payroll tax cut must remain in effect.”

In the past three years, Mr. McCain has been a harsh critic of Mr. Obama. His earlier reputation, as a Senate moderate who often challenged the Republican ideology, has been less visible of late as he has repeatedly sided with Mr. Boehner and House Republicans against the president.

But Mr. McCain voted for the two-month extension and urged fellow Republicans in the House to “figure out a way through this” to make sure that taxes do not rise next month.

“I think we have to recognize reality, and that is that we are not going to see the payroll tax cut expire on the first of January, and we have to accommodate to that reality,” Mr. McCain said. “It would not be fair to the American people at this time.”

Mr. Boehner on Tuesday urged Mr. Obama to direct the Senate to appoint negotiators to work out a deal on the yearlong extension of the payroll tax cut before the end of the year.

In a news conference, Mr. Boehner assailed Mr. Obama and Congressional Democrats the way he has done repeatedly throughout the last year.

“All of this needs to be done in the right way,” Mr. Boehner said. “I’ve seen Congress kick the can down the road, kick the can down the road. It’s time to stop the nonsense.”

But for the first time since he became speaker, Mr. Boehner’s criticism is aimed not just at a partisan majority in the Senate, but at his own Republican colleagues.

Will that breakdown of unity continue as Election Day 2012 gets even closer? Or will Republicans in the two chambers find a way to once again stand shoulder-to-shoulder against Mr. Obama’s agenda?

The answer could help to determine the fate of Mr. Obama’s re-election and the future shape of the Congress.