AS a general matter, the party that holds the White House over two terms does poorly in the second midterm election, presenting one clear challenge to red-state Democratic senators on the ballot in 2014.
However, a number of these senators will face an additional obstacle: their own records.
Sens. Mark Pryor, Ark., and Mary Landrieu, La., are two prime examples.
They voted for Obamacare and the first-term stimulus. Last week, they were two of only 20 senators who voted against repeal of the medical-device tax, which will hit everything from wheelchairs to pacemakers and boost costs for many more patients than just the "rich."
Another pivotal moment for these senators came early Saturday morning, in a vote on a budget supported by Senate Democratic leaders - a mammoth tax increase lacking any real entitlement reform.
Landrieu voted for it; Pryor voted no, as did three other Democrats who face tough reelection races in 2014: Kay Hagan, N.C., Mark Begich, Alaska, and Max Baucus, Mont.
The budget passed by the narrowest of margins, 50 to 49.
As the Democratic Party abandons its more moderate constituents, these senators are facing an ever more difficult situation, reconciling the demands of the Democratic Party and those of their home-state voters.
It is one thing for them to have run in 2008, an historic presidential election year when every Democrat in sight turned out; it is quite another to run in an off-year election, tied to a president whose approval rates are sinking.
Votes on the budget, anti-gun legislation, energy development and other issues that resonate back home will test just how much they are willing to risk in support of their party leadership and the White House.
These candidates and others ran on the promise of independence and serving their more conservative constituents. If they haven't delivered and if the GOP fields capable opponents (unlike as in many 2010 and 2012 races), their political future will be at risk.
Rubin is a blogger for The Washington Post.