It was encouraging on Monday night to hear the Democratic candidates arguing in South Carolina over the virtues of Ronald Reagan. The Gipper has already cast a long, dark shadow over the Republican 2008 presidential race as each candidate in turn has claimed to be the true keeper of the Reagan flame. It has been a largely destructive process.

As the conservative candidates have discovered, laying claim to Reagan’s mantle leads to voters making unfortunate comparisons. Reagan’s great skill was to catch the tenor of the national narrative, to inspire Americans with his optimistic vision, and to focus upon delivering a few simple but important policy goals.

Those talents do not reside in a single Republican candidate. One is as handsome as Reagan, some have had as many wives as Reagan, and one is as old as Reagan, but none have the special combination of qualities that made Reagan one of America’s most successful and beloved presidents.

Reagan’s greatest achievement, perhaps, was that he changed the political weather, for both parties, as Barack Obama’s rare moment of candor to the Reno Gazette-Journal editorial board testified.

The senator suggested that Reagan had “changed the trajectory of America in a way that Richard Nixon did not and in a way that Bill Clinton did not. He put us on a fundamentally different path because the country was ready for it.” It was hardly an original thought, but Democrats are not used to hearing one of their own say good things about a Republican president, so it caused a stir.

Mr. Obama risked the wrath of the great majority of Reagan-hating Democrats because he sees himself as another Reagan. The message of hope he has been peddling in his ponderous way relies upon a great leap of imagination. An America with President Obama in the White House, he suggests, would be a country that had turned its back on the deep political, social, and racial divisions that feed the current political debate. It is a tall order, but Mr. Obama knows that only those with the highest ambitions get to change anything.

It is in that spirit that Mr. Obama added, “I think it’s fair to say that the Republicans were the party of ideas for a pretty long chunk of time there over the last 10 to 15 years, in the sense that they were challenging conventional wisdom.” Reaganism gave way to the “New Democrat” ideas of the Clintons, then to George W. Bush’s neo-conservatism. Mr. Obama now believes it is his turn to grasp the national narrative and that there are enough young idealistic voters, and enough unreconstructed progressives among the baby boomers, to allow him to chart new territory.

Mrs. Clinton’s response on Monday led to one of the most aggressive debate tussles so far: “He has said in the last week that he really liked the ideas of the Republicans over the last 10 to 15 years, and we can give you the exact quote.” The “exact quote” shows that Mr. Obama said nothing of the sort. Nonetheless, the Illinois senator found himself, not for the first time, caught in a painful spat with Mrs. Clinton. As Rep. Jim Clyburn advised Mr. Obama, a little too late: when calling Ronald Reagan in his defense, he would have done better to distance himself from the great man’s policies.

Mr. Obama has a lot to learn about politics. Which is why he made a cardinal mistake in the Las Vegas debate in confessing his greatest weakness. “My desk and my office doesn’t look good. I’ve got to have somebody around me who is keeping track of that stuff. And that’s not trivial. I need to have good people in place who can make sure that systems run.”

Again, Mrs. Clinton was quick to pounce. Governing isn’t about high falutin’ words and getting others to implement the policies, she said, it is about rolling up your sleeves and working hard. “I respect what Barack said about setting the vision, setting the tone, bringing people together. But I think you have to be able to manage and run the bureaucracy,” she said.

Margaret Thatcher would agree with Mrs. Clinton. As prime minister, she found it difficult to delegate and was always peering over her cabinet colleagues’ shoulders to ensure they were following her commands. Nothing was too small to escape her notice. Once she reprimanded Nigel Lawson, her most successful chancellor, demanding, “Isn’t it about time you had your hair cut?”

In Reagan, however, the tidy minded Lady Thatcher found a soul mate, albeit one who, like Mr. Obama, needed a team of competent executives around him to turn his vision into reality. The pair were yin and yang, he the good cop, she the bad. Yet when working with Reagan, Lady Thatcher often found his muddled modus operandi frustrating. Their private letters and telephone conversations attest to her shortness with him when she thought he had lapsed.

But Reagan’s strength, as Lady Thatcher often acknowledged, lay in the way he sold his policies to the world. He made enormous changes, like the end of Soviet communism, seem attainable and inevitable. Senator Obama’s problem is that, for all his soaring words and cadences, his vision remains incomplete and implausible.

Martin Luther King’s dream pervaded Monday night’s debate, with Senator Obama making the silent appeal that this is the right time for an African-American to be a presidential candidate.

Mrs. Clinton made clear this week that if voters want Dr. King’s vision to be completed, she is ready for the task. If Senator Obama is painting a new world way beyond Dr. King’s “promised land,” he is running out of time to spark the nation’s imagination.

There is little evidence in the primary results thus far to suggest that “the country is ready for it” and that voters in November will be prepared to take a leap in the dark for a misty eyed glimpse of utopia.

Mr. Wapshott’s “Ronald Reagan and Margaret Thatcher: A Political Marriage” is published by Sentinel.