Here are the two most prominent people in presidential politics three summers away from the national nominating conventions: a Republican who very likely can win the general election but likely can’t win his own party’s nomination, and a Democrat who very likely can win her party’s nomination but likely can’t win the general election.

The pronouns provide a hint, but even if they didn’t, you would probably guess that we’re talking about Sen. John S. McCain of Arizona and Sen. Hillary Rodham Clinton of New York. They’re not as unalike as you think.

They’re not really from the states they represent. They’re not really in the mainstream of their own parties. They’re not really what they seem to be.

But they are the most intoxicating figures in American politics today – and together they personify the problems the political establishment has as the 2008 campaign approaches, and underline the power the conservative movement possesses.

The Republicans were once so flexible that, in modern times, they six times chose nominees who were simply less extravagant exponents of many of the very same policies their opponents espoused. (The six: Dwight Eisenhower twice, Richard M. Nixon three times, Gerald R. Ford once.) They won four of those six elections, a pretty good record. But they won five of the next seven with true-blue conservatives (including, for the sake of this argument, President Bush’s father, whose heart was moderate but whose governing style was mostly not).

As a result, the Republicans are now a more ideological political party in the post-Reagan era than the Democrats were in the post-Roosevelt era.

The Democrats, once the party of daring and conviction, now are the party of dithering and doubts. Their most successful figure of the past generation, Bill Clinton, was a centrist who worried about the bond market and signed a welfare bill that Sen. Daniel P. Moynihan, the architect of the Nixon welfare philosophy, deplored as punitive and heartless. The party that once rang the bell of freedom now reflexively wrings its hands over its own problems and its own lack of identity.

Now the two parties have no natural leaders.

George W. Bush, so tentative and unsure as a presidential candidate in 2000, is the defining political figure of the times, so much so that the Democrats seem to exist merely to assert their hatred of him. The Republicans are so in his thrall and control that no logical successor figure has emerged. (The natural contrast is how Al Gore was always regarded as the logical successor to Clinton. Dick Cheney’s disavowal of presidential hopes denies the Republicans the kind of choice they like to make: grabbing the next senior statesman.)

Now back to Clinton and McCain. They’re constantly in the news, constantly on television, constantly the subject of buzz. McCain was the hero of an A&E cable movie, based on his autobiographical “Faith of Our Fathers,” that emphasized his courage as a prisoner of war during the Vietnam era. Clinton is the subject of a new biography so virulently negative that her customary opponents have shunned it for fear of being soiled by the muck. At the same time, nearly four dozen Hillary for President items, including baseball jerseys, dog T-shirts, thong underwear and an infant creeper, are available on the Internet.

The two also are constantly redefining themselves.

McCain, the darling of the Democrats, nonetheless is an abortion opponent and an affirmative-action skeptic. He opposes big business’s influence on politics through campaign expenditures but doesn’t necessarily oppose big business’s viewpoints.

Few political passages have been chronicled with the intensity brought to bear on that of Clinton. She has nudged away from the big liberal women’s groups’ positions on abortion, softened her edges on health care, and even supported President Bush in his decision to go to war with Iraq, placing her closer to the center than Sen. John F. Kerry of Massachusetts, the party’s 2004 nominee. Here is a telling sentence from a lengthy New York Times examination last week of her Senate record: “On abortion, she can sound like a supporter or an opponent, calibrating the tone and nuance of her remarks to suit the audience she faces.”

McCain’s problem is within his own party: Republicans view him as an apostate, a reluctant (and insincere) collaborator with the president, an unseemly populist, a shameless camera hog and a stealth liberal. Clinton’s problem is outside her own party: Her opponents regard her as an unreconstructed liberal, an unswerving enemy of business, a radical feminist and a shameful enabler of a shameless spouse.

This analysis underscores the critical role that conservatives now play in our politics. They stand athwart McCain’s road to the nomination and athwart Clinton’s road to the White House. It is a position, and a power, liberals have seldom possessed.

That said, McCain and Clinton retain the whip hand in the nomination fights. They’re the best-known, and are likely to be the best-funded, presidential aspirants in a still-forming field. They’re the man and woman to beat.

If the state of the nation is parlous, then no Republican would have a lock on the White House, and Hillary Clinton might win the presidency. If the economy goes rancid, or the war lingers on, or terrorists continue their rampage, the Republicans might turn to the candidate they think would have the best chance of winning the election. That’s the reason George W. Bush won the nomination in 2000, and the supreme irony of it all could be that John McCain could win the White House for the very same reason his bete noire did eight years earlier.