This week, large numbers of Republicans, led by Sen. John McCain (R-AZ), broke with McConnell and voted with Democrats to secure the confirmation of controversial Obama nominees to the Labor Department, Environmental Protection Agency and Consumer Financial Protection Bureau. In all eight cloture and confirmation votes, McConnell voted "no."
The most controversial nominee so far, Tom Perez for labor secretary, overcame a GOP filibuster by the thinnest of margins, 60-40. The six Republicans who joined Democrats in his favor, whom Democrats will look to for cooperation on other matters, were Sens. McCain, Bob Corker (TN), Lamar Alexander (TN), Susan Collins (ME), Mark Kirk (IL) and Lisa Murkowski (AK).
In a clear sign of boiling rank-and-file frustration, Corker reportedly cried "bullshit" loudly while McConnell was discussing the issue of nominations and Democrats' nuclear option threat during a closed-door GOP meeting on Wednesday. He later declined to apologize for it and said he's "glad that that occurred."
On immigration, 14 Republicans joined every Democrat in voting to comprehensively overhaul the system and offer unauthorized immigrants a path to citizenship.
On the budget, numerous Republican senators are urging conservative colleagues to stop blocking conference negotiations with the House, and are pushing for a long-term budget agreement with Democrats that includes new revenues -- anathema to the tea party.
McCain has led the dissent in each of these cases, earning effusive praise from leading Democratic senators and prompting jokes this week by Democratic aides that he is the new minority leader.
John Sides, a political scientist at George Washington University, pointed to three apparent factors in the divisions: "[S]incere preferences about policy," the outcome of the 2012 election, and the fact that some senators "value the institutions of the Senate."
"They want to Senate to work better than it has been, and believe that confirming presidential nominees is part of that," he said. "And they also value the filibuster, too. They want to preserve that feature of the Senate, and so a compromise on nominees was better than Reid's using the nuclear option."
Complicating matters for leadership is that McConnell and his No. 2, Sen. John Cornyn (R-TX), are both unpopular at home and face reelection next year. As a result, they're working to ward off primary challengers by voting against Democratic initiatives as much as possible and avoiding the appearance of working with President Obama. That makes it harder for them to balance the concerns of rank and file members, who watched their party get crushed in a second consecutive presidential election and aren't eager to spend another four years obstructing.
But it remains to be seen whether the divisions will usher in a new era of Senate cooperation, as McCain strikes a conciliatory posture with his 2008 rival on upcoming battles involving the debt ceiling and nominees to the influential D.C. Circuit Court of Appeals.
On nominations, McConnell had sought to so vastly redefine the parameters of a Senate minority's obstructionist muscle that the beating he took this week ultimately amounts to a battle lost in a war he's still comfortably winning. On immigration, senior Republicans tacitly gave reform their blessing, seemingly for the sake of the party, even as they voted against the legislation. On the budget, the larger GOP divisions are between the Senate (where members are less enthusiastic about massive spending cuts) and the House.