Kohut’s analysis avoids the biggest factor expediting Democratic polarization: the president’s health care law. Obama entered office with a near-filibuster-proof Democratic supermajority in the Senate and his party holding 59 percent of seats in the House. Despite widespread opposition, he spent immense political capital to pass health care reform, which depleted his party’s moderate Congressional wing. The Democrats who retired or lost reelection in the 2010 cycle disproportionately hailed from the party’s middle. In just four years, the number of moderate Blue Dogs shrank from 54 to 19 members—with three more retiring this year, and at least five others facing tough reelection campaigns.

Meanwhile, in an election year where control of the Senate is hanging in the balance, the White House is ignoring ways to mitigate damage for his party. Approving the Keystone XL pipeline would help many of his party’s most vulnerable senators, but the administration has indefinitely delayed a final decision. The White House’s controversial nominee to head the Justice Department’s civil-rights division, Debo Adegbile, was rejected by seven Democratic defectors, including two red-state senators on the ballot in 2014. Obama’s 2014 campaign strategy to energize the base could help turn out African-American voters in a couple of Senate battlegrounds (North Carolina, Louisiana), but it’s a sign he’s already given up on persuading white moderates in Republican-friendly states.

Indeed, in understanding the challenges vulnerable Democrats face in 2014, it’s worth recalling how differently Obama has approached his second term compared with Bill Clinton.