The Senate’s dynamic was on display in a series of amendment votes over the last week on the Keystone XL oil pipeline: New Hampshire Sen. Kelly Ayotte, one of the Democrats’ top targets in 2016, repeatedly broke with her party, including by opposing efforts to hamper Obama’s international climate negotiations. Illinois Sen. Mark Kirk was the lone Republican to back regulations on petroleum coke, a byproduct from oil refining. And conservative Pennsylvania Sen. Pat Toomey was one of 15 Republicans to endorse a statement that humans contribute to climate change.

Another defector, North Carolina Sen. Richard Burr, was the only Republican to vote for a Democrat-backed mandate that the pipeline be built with U.S. steel. “It’s real simple,” he said proudly. “I’ve been for ‘Buy America’ every time it’s come up in the Senate.”…

If McConnell goes too far to the right, he could lose swing-state GOP senators who need to moderate on issues that resonate in their backyards, especially during a presidential election year that’s expected to bring a surge of Democratic turnout. But if McConnell tries to move to the center on issues that play well in Ohio or Wisconsin — where GOP Sens. Rob Portman and Ron Johnson face reelection, respectively — he could have a hard time reconciling the Senate’s legislation with what comes out of the conservative House.