That was the easy part. Taking over Congress, that is. Let’s face the truth here: Scoring a triumph over Republicans who themselves were impatient with the Republican record during an unpopular war was no great achievement, despite the great deal of celebrating that it prompted among Democratic partisans. Indeed, the startling thing would have been if, under these circumstances, the Democrats hadn’t prevailed.

But they did, and though only a few days have passed since the big moment, a sobering wave should be passing through Democratic ranks right about now. They have the Congress, the great prize. But power brings responsibility, and even in a system that permits and perhaps encourages divided government, the burden now passes to the Democrats. They have to do something with that power besides reward themselves with chairmanships and patronage and the psychic spoils of office.

That is the hard part, and when the Republicans pulled the same trick a dozen years ago, in 1994, they had an enormous advantage. They had, besides four decades’ worth of seething resentment over slights small and large, a roadmap. They had the Contract With America, which was really a contract with themselves.

The Democrats have a plan for the first few days, and much of it involves getting sworn in and swearing not to do as the Republicans have done. That is not good enough. If they are looking at their 2006 victory as a staging ground for 2008, which is the political way of doing things, they are going to maneuver the president into one uncomfortable corner after another for the pure recreational value of it all. If, on the other hand, they look at last week’s victory as a chance to change the country, there’s going to be a lot less recreation but perhaps some value.

So the first thing they have to do is to choose between those options. Do the purely political and make the president look powerless at a time when American power is being tested in Iraq? Or do the responsible thing and try to govern the country as if the nation had said something Tuesday besides indicating that it was weary of the guys who have been in power for six years and who, tired and tendentious, seem to have run out of gas.

If they take the latter route — if they try to govern from their beachhead in Congress — then they will have to do something more than simply say no to the president.

No longer is it good enough to say that the president prevaricated getting us into Iraq and bumbled once we got there. Now the Democrats have to say how they would win or leave. No longer is it good enough to say that the president’s plans for Social Security are a radical departure from the 1935 vision of Franklin Roosevelt. Now the Democrats have to say what they would do to keep the system secure, or whether they would change public expectations of how much help Social Security will provide Americans when they retire.

These two choices — win or leave, strengthen Social Security or cut public expectations and public disbursements — may seem stark. But those are the choices. Neither problem, both signature challenges of our era, is ripe for fuzzy responses or fuzzy math. Choose one, make the argument, take a vote, live with the consequences.

There are scores of other choices like those. The war on terror, which no one wants to abandon. Climate change, which every industrial nation but America has at least tried to address. Education, where every problem cannot be solved by having students take tests and having teachers teach to those tests. Competitiveness, a word from the 1980s, perhaps, but a challenge for the 21st century. The tax system, which hasn’t been overhauled for 20 years and isn’t getting any more rational. The balance between religious life and civic life, a challenge Americans have struggled with for two centuries, but which seems even more stubborn in an age of rapid technological change.

One thing no one, including the ascendant Democrats, says about President Bush is that he is unwilling to make a choice and live with it. A short time after September 11, 2001, Thomas Rath, the New Hampshire political strategist, encountered the president at a White House event. Mr. Rath is a congenial man, his impulse unfailingly to offer comfort. To Mr. Bush, like the rest of the country still dealing with the immediate shock of the terrorist attacks on Washington and New York, he said something along these lines: “Nobody asks to deal with something like this.” Mr. Bush responded immediately and forcefully: “Some people do.”

So the next time you see congressional leaders wringing their hands at the difficulties posed by Social Security or Iraq or whatever the next crisis is, remember that encounter between Messrs. Rath and Bush. It isn’t as if nobody asked to deal with problems like those the newly empowered Democrats face. They asked.

Permit one more presidential anecdote to make the point that power carries great and grave responsibility. The setting for this story is Washington, the time just after the death of President Roosevelt in April 1945. Harry Truman, the new president, offered his assistance to Eleanor Roosevelt, newly widowed and, we now know, humiliated by the way her husband died, at the side of Lucy Rutherfurd, his long-ago lover. She said there was nothing, saying instead to Mr. Truman: “Is there anything we can do for you? For you are the one in trouble now.”