Nearly complete delegate returns from coast-to-coast races on Super Tuesday left McCain with 703 delegates, nearly 60 percent of the 1,191 needed to win the nomination at the convention in St. Paul, Minn., this summer. Former Massachusetts Gov. Mitt Romney had 269, former Arkansas Gov. Mike Huckabee 190 and Texas Rep. Ron Paul 14.

Despite the lengthening odds, Romney and Huckabee showed no clear signs they were ready to exit the race.

The Democratic delegate count lagged, the result of party rules that shunned the type of winner-take-all primaries that helped McCain build his advantage.

On Tuesday's busiest primary night in history, Clinton and Obama were separated by 40 delegates, with several hundred yet to be allocated.

Overall, that left Clinton with 1,000, and Obama with 902, neither of them even halfway to the 2,025 needed to secure the Democratic nomination.

With little time to rest, both pointed toward the next contests, primaries in Louisiana, Maryland, Virginia and the District of Columbia plus caucuses in Nebraska, Washington Maine and the Virgin Islands over the next week. In all, those states offer 353 delegates.

At a news conference in Chicago, Obama claimed victory on Super Tuesday, saying he had won more states than the former first lady for the day and would wind up with more delegates by the time all were tallied.

He bluntly took issue with the suggestion that he, more than she, could be brought down by Swift Boat-style criticism in the fall campaign.

"I have to just respond by saying that the Clinton research operation is about as good as anybody's out there," he said.

"I assure you that having engaged in a contest against them for the last year that they've pulled out all the stops. And you know I think what is absolutely true is whoever the Democratic nominee is the Republicans will go after them. The notion that somehow Senator Clinton is going to be immune from attack or there's not a whole dump truck they can't back up in a match between her and John McCain is just not true."

He said he would be campaigning in all the states in the next round of primaries and caucuses.

Clinton's plans were not yet public for the next contests, and senior aides conceded Obama would have more to spend on ads.

"We will have funds to compete," Mark Penn, Clinton's chief strategist, told reporters in a conference call. "But we're likely to be outspent again."

Asked whether Clinton and her husband, the former president, had decided to dip into their own wealth to finance their campaign, Penn said, "I'm not aware."

But a few hours later, Howard Wolfson, the campaign's communications director, said the senator had loaned money to her campaign late last month.

Officials with both campaigns have said Obama raised $32 million in January and that Clinton raised $13.5 million, a significant gap between the two that allowed Obama to place ads in virtually every Super Tuesday state and to get a head start on advertising in primaries and caucuses over the next week.

The delegate math was far different for the Republicans, and so, too, the race.

Inevitably, there was speculation among Republicans that Romney was contemplating a withdrawal. He offered little hint of his plans, announcing only that he would appear before conservatives and make a speech to Maryland Republicans on Thursday.

The wealthy former businessman has spent an estimated $40 million to $50 million in personal funds to finance his campaign, but he ran something of a scaled-back effort in several Super Tuesday states.

He emerged with a string of victories in caucuses, but won primaries only in his home state of Massachusetts and in Utah.

Huckabee scored a series of Southern victories on Super Tuesday and has said he will stay in the race until someone has enough delegates to clinch the nomination.

Interviewed on CBS, Huckabee sidestepped when asked whether he might be an irresistible vice presidential running mate on a ticket headed by McCain. "I still want to be the irresistible choice to be the president," he said.

McCain wasn't talking on that subject, but the vote totals and exit polls made it abundantly clear that he was weak where the former Arkansas governor was strong - in appealing to evangelical conservatives in the Bible Belt.

A Louisiana primary and caucuses in Kansas on Saturday offer an inviting target for Huckabee, who has demonstrated a strong appeal to social conservatives.

McCain didn't say so, but there is ample evidence he is pleased to have Huckabee in the race. In state after state, Huckabee has divided the anti-McCain vote with Romney, preventing the former Massachusetts governor from emerging as a more serious threat.

On Tuesday, McCain's delegates at the West Virginia convention swung over to support Huckabee at the last minute in a successful maneuver designed to deprive Romney of a victory.