People seeking high office dream of the great good they will do, the problems they will end, the wisdom they will bring to bear on the direction of policy and the decency they will inject into public discourse.

Out in the cold with their noses pressed against the glass, they can see the levers of power but cannot tell how difficult it is to pull them. Once politicians gain office, they discover that they must learn the art of the possible. They learn, too, if they are very lucky, that the perfect will be the enemy of the good. (More likely, however, they learn that the good and merely mediocre are enemies of the lousy — which is what they must settle for.)

When President Bush won reelection in 2004 and his party retained majorities in the House and Senate, Republicans overplayed their hand. Their political capital ran out by the time the 2006 midterm elections arrived, and the GOP was handed a thumping defeat that even some its members acknowledge was deserved.

Their debacle was the Democrats’ triumph; Speaker Nancy Pelosi (D-Calif.) and Senate Majority Leader Harry Reid (D-Nev.) led their troops out of the wilderness and into the majority, arriving at the summit of Capitol Hill justifiably elated and with expectations high.

Now, however, they are leaving for Christmas thoroughly subdued. They have achieved more than nothing — a higher minimum wage, a student loan bill, an ethics bill — but they have also failed to secure some of their highest priorities.

They have not changed administration policy in Iraq and, lower on the scale of hoped-for outcomes, they have not managed to improve the appropriations process or craft a veto-proof extension of the State Children’s Health Insurance Program.

One of Pelosi’s impressive achievements in the 109th Congress was to keep her caucus united. She showed what a minority can achieve against a fractured majority party; she made it tough for the GOP to get much done or look good.

Now the Democrats are being forcefully reminded that two can play at that game. Republicans in both the Senate and House refuse, for example, to raise taxes to offset the “patch” on the Alternative Minimum Tax and have forced the Democrats to abandon the pay-go rules with which they hoped to entrench a reputation for fiscal responsibility.

 No one wants to be in opposition, but, ah, the travails of power.