It is common these days in Washington to portray the Republican Party as facing potentially existential dangers stemming from the outlandish influences of its Tea Party contingent, its strident opposition to President Obama’s liberal leadership, and underlying demographic trends in the country. And it is true that the Republicans face serious challenges, internal and external, that aren’t going away anytime soon.

But the Republican Party isn’t the party most in danger of a political blowout over the next three years. That distinction belongs to the Democrats, and it can be seen in a fundamental reality of America’s two-party system—namely, nothing devastates a ruling party as surely as a failed presidency.

This is not to say that Obama’s second term will be a failure. It’s too early for such a judgment. But the signs of failure are evident, perhaps most clearly in the perceptions of congressional Democrats facing reelection next year. It appears that some are beginning to put daylight between themselves and their president. Louisiana senator Mary Landrieu flew to her home state with Obama on Air Force One last week, but once they landed, according to the Wall Street Journal, she was too busy with other commitments to accompany the president during his state visit. The paper also reported that Obama met recently with Senate Democrats facing reelection and heard complaints about the rollout of his healthcare legislation and its impact on the nation’s political climate....