All power is fleeting, especially political power. A sober analysis from Phil Klein:

The American public didn't go from being socialists to Reaganite conservatives in the past two years, any more than their ideology radically transformed from 2004 to 2006. The lesson of recent elections, thus, may not be that the American people are right of center, or left of center, or dead center, but that many of them aren't terribly ideological. This means that political power is ephemeral. No matter how popular one party is, they could be only one election away from embarrassing defeat. No matter how badly one party is defeated, they could be on the verge of a historic comeback. In this environment, reports of the demise of any political party, at any time, are likely to be greatly exaggerated.