There was good news and bad news for John McCain in his final debate with Barack Obama. The good news was that he got off the best line of the night. The bad news was that it helped Obama drive home his strongest argument against him.

When Obama began his familiar riff tying McCain to the unpopular incumbent of the Oval Office, McCain shot back: "Sen. Obama, I am not President Bush. If you wanted to run against President Bush, you should have run four years ago."

Obama didn't miss a beat in replying: "The fact of the matter is that if I occasionally mistake your policies for George Bush's policies, it's because on the core economic issues that matter to the American people — on tax policy, on energy policy, on spending priorities — you have been a vigorous supporter of President Bush."

In that one exchange, McCain unwittingly enabled Obama to capsulate his drumbeat pitch — that a McCain election would amount to a Bush third term.

To McCain's credit, he did his level best over the 90 minutes of the most intensive and best-moderated debate of the campaign to raise questions about Obama's fitness to be president. He threw a combination of policy and personal punches at his foe but was unable throughout to shake Obama's determined poise.

While keeping the Illinois senator on the defensive through most of the debate, McCain never scored the kind of verbal blow he needed to change the complexion of the race. Thanks to moderator Bob Schieffer of CBS News, who deftly and persistently kept the candidates on track, they engaged in serious exchanges throughout.

As promised, McCain confronted Obama with his association with William Ayers, the Chicago lawyer who in the Vietnam-war era was part of the Weather Underground terrorist group. He said "I don't care about an old washed-up terrorist," but "we need to know the full extent of that relationship."

An unruffled and prepared Obama brushed the matter aside by saying he had condemned Ayers' conduct at the time as "despicable acts," then pivoting to a charge of his own — that the Ayers matter was a diversion from the real issues facing the country. "The fact that this has become such an important part of your campaign, Sen. McCain," he said, "says more about your campaign than it says about me."

In raising the Ayers association, McCain was responding to demands from many fellow Republicans that he do so. But he ignored their urging, shared by running mate Sarah Palin, that he also confront Obama over his other controversial association, with former pastor Jeremiah Wright. McCain earlier had vowed he would not do so, and he stuck to his guns.

This exchange aside, the third presidential debate produced a welcome civility and substance, as McCain strove aggressively to find a chink in Obama's cool demeanor and persistent hammering of his argument that McCain's election would mean more of the same policies of the last eight years under Bush.

On the critical issue of the financial crisis that has dominated the late stages of the presidential campaign, both candidates merely sparred on their separate proposals to bring relief to middle-class voters facing home foreclosures and deprivation of healthcare.

McCain's charge that Obama in his healthcare plan was engaging in "class warfare" by proposing to "spread the wealth around" through heavier taxation of the wealthy was a tired recitation of an old conservative lament worthy of Bush himself.

As in the first two debates, the third was not the game-changer that political strategists and news-media referees had been arguing McCain needed to shatter the narrative that Obama was building and widening a lead toward his election on Nov. 4. In that sense, for all of McCain's heightened aggressiveness, the challenge remains the same for him — to make an effective argument that newcomer Obama is not ready to be president.

Seeing the two men side by side in three grueling debates, with McCain's desperation increasingly visible, was itself telling in his need to make that case.

With less than three weeks to go in the campaign, there is nothing apparent on the horizon right now to help McCain cut Obama down to size.

Jules Witcover's latest book, on the Nixon-Agnew relationship, "Very Strange Bedfellows," has just been published by Public Affairs Press. Reach him at juleswitcover@earthlink.net.