The 1946 elections were a Democratic disaster. The Republicans came up with an irresistible two-word slogan (“Had enough?”) and the Democrats, facing the voters without Franklin Roosevelt seeking or serving in the White House for the first time in 14 years, lost 11 Senate and 55 House seats.

We think today of Harry Truman as a no-nonsense man whose plain-speaking style won America’s heart, but in real time Truman was a lot less winning and beguiling than he is in retrospect. Truman was hurt, and he felt defeated, and he sent his wife, Bess, one of the bluntest letters ever written by a president:

“I’m doing as I damn please for the next two years and to hell with all of them.”

That was 60 years ago. Truman went on to have a fairly successful final two years of his term – and then he won a second term. President Bush isn’t in that position; he’s already served nearly six years and is barred from seeking another term. But he’s not barred from saying to hell with all of them and vowing he’ll do as he damn pleases for the next two years.

But it’s not likely he will – or get away with it. Mr. Bush isn’t Truman, and America in 2006 bears almost no resemblance to America in 1946. Sixty years ago, America had completed a war; today, we’re in one. Sixty years ago, there was a broad consensus about how to fight communism; today there is no broad consensus about how to fight terrorism. Sixty years ago, the press was tame by contemporary standards; today’s press, though not as robust economically as it was in 1946, is in fighting trim philosophically and isn’t about to relinquish its watchdog role in American politics.

So Mr. Bush may proclaim in private that he’ll do as he darn pleases – Mr. Bush is a bit more restrained in his rhetoric than Truman was, and there’s no evidence he’s ever been tempted to punch a reporter in the nose, as Truman was – but he’s got a lot of restraints. They’re instructive, for they tell us a lot about the political culture we have today, and they provide hints about the amount of latitude Mr. Bush has, even in a period when people are speaking openly about a nuclear attack against Iran.

Only a decade ago a columnist writing a piece like this would cite the War Powers Act, which ordinarily restricts presidents from undertaking adventures abroad, or at least restrains them from keeping such adventures going on for very long. But Mr. Bush has almost rendered the War Powers Act (reviled by all of his predecessors since it was passed in the Nixon years) irrelevant with his doctrine asserting that the president has the right to take pre-emptive action in situations where American national security is at risk. This may be the most profound legacy of the Bush years, debated bitterly in Congress and in political science seminars for a generation.

The act still remains on the books, but even in its days of relevance it couldn’t have prevented President Reagan or Clinton from undertaking brief forays (in Grenada, in Afghanistan) in trouble spots where a president saw mischief brewing. If the president was right, the thinking went, he’d get away with just about anything. But then again, Mr. Reagan wasn’t exactly right in Grenada and Mr. Clinton wasn’t exactly on target in Afghanistan, and neither paid any price for their initiatives.

Even so, there are some powerful restraints on presidential prerogatives, and most of them are derived from the source that, ironically enough, often provides presidents their greatest latitude: moral power. Now it may seem silly, or antiquarian, to speak of moral power in an age of immorality, but it is at work every day in our politics. In truth, Mr. Bush’s abuse of moral power – or, perhaps, his abuse of what he believed was his moral power – in talking of weapons of mass destruction in Iraq got him his way in 2003 but could prevent him from getting his way in 2007 and 2008.

The moral power that is at work now is the moral powerlessness of the boy who cried wolf. Mr. Bush called WMD and got away with it. If he tries it again, he won’t. The Democrats will delight in citing President Reagan’s favorite Russian aphorism: Doveryay, no proveryay. In English: Trust but verify.

Indeed, there is a lot the president cannot do in his last few years. He can’t come up with a big-vision spending program; he’s spent too much, on homeland defense and the war in Iraq, and, besides, the Republicans won’t let him spend any more. He can’t come up with a big-idea proposal on the biggest domestic problem he has, the future of entitlements like Social Security; he already came up with one and it was a giant dud. He can’t come up with a big-hearted idea on immigration; his own party is divided like mad on the issue, and it has signaled that it isn’t interested in presidential leadership on it. He’s boxed in.

He may alter the cast of his administration – the substitution of Josh Bolten for Andrew H. Card Jr. may be the first step – but he cannot alter the character of the administration, nor can he change the borders of acceptable behavior that have been set for him by the very record of his own administration. He has to stay the course, if only because that is the only course open to him.

He’s not the only president to feel this sort of restriction. Bill Clinton was forced by his own personal behavior to stay the course in his last two years; impeachment and Senate trial left him little recourse. Ronald Reagan was forced by Iran-Contra to stay the course; he remained the Great Communicator, but his mastery of his own White House was never taken as an assumption again.

So in the end, Mr. Bush cannot do whatever he damn pleases. He’s the president, but the rhythm of politics and the trajectory of his own presidency suggest that, from here on in, he lacks the power to project much of the power of his office.