WASHINGTON (AP) – This time, John McCain kept Barack Obama on the defensive.

The feisty Republican tried hard to find a lifeline Wednesday night, challenging his Democratic rival at every turn over his truthfulness, associations and record.

By that measure, McCain won the last debate of the 2008 campaign.

But that may not be enough.

McCain still desperately needs to change the trajectory of a race that's tilting significantly toward Obama. Democratic voters outnumber Republicans, the economic crisis has transformed the race in
Obama's favor, President Bush is extremely unpopular, most voters think the country is on the wrong track and the Democrat is leading in key-state polling.

There's little McCain can do on his own to change the dynamic. Not that he didn't try. "I am not President Bush," he said.

"You didn't keep your word," McCain reminded Obama on the issue of accepting public financing, something Obama had said he would do if the GOP nominee followed suit.

McCain, seemingly more prepared and definitely more aggressive than in past debates, called Obama's tax plan "class warfare," accused him of failing to stand up to his party's leaders and said the Democrat twisted his record in ads.

With his repeated attacks, the Republican ran the risk of turning off voters. Their negative impressions of him have risen as he questioned Obama’s character over the past week. A New York Times/CBS News poll this week found that more voters see McCain as having waged a negative campaign than Obama.

Yet, McCain had little choice but to turn up the heat – and endure the consequences.

With the election in less than three weeks, the debate season is over and there are no more high-profile opportunities that can guarantee McCain an audience of tens of millions of people. Flush with cash, Obama has bought 30-minute blocks of prime-time advertising six days before the election; McCain may not be able to afford the same.

Over the next 20 days, both candidates will go after the voters who say they could still change their minds.

There are a lot of them – about one-third of all voters – but McCain has to win many more than Obama. Not only is he behind in the polls, but the base of all-but-certain Republican voters is smaller than Obama's Democratic foundation.

The Arizona senator's challenge is great.

In the race for 270 Electoral College votes, Obama has comfortable leads in polls in Democratic-held states and is competitive if not ahead in surveys in Republican bastions.