WASHINGTON (Reuters) - Barack Obama and Hillary Clinton on Wednesday faced a long fight for the Democratic U.S. presidential nomination, with Obama promising to get tougher on his rival and Clinton hinting the two could team up in November against Republican John McCain.

McCain won the endorsement of President George W. Bush in a White House ceremony that capped the Arizona senator’s rise to the party’s nomination after his campaign nearly collapsed last year.

While McCain turned his attention to the November election, Democrats faced more of the grueling state-by-state nominating duel between Obama and Clinton that threatened to extend into June or beyond.

Clinton dodged another possible knockout blow from Obama on Tuesday, coming back from a series of losses to win the big states of Ohio and Texas and revitalize her campaign. The New York senator said the wins showed she had the type of broad support needed against McCain.

Obama promised to more aggressively confront Clinton, who hammered him in Ohio and Texas with questions about his readiness to be commander in chief, his commitment to renegotiating U.S. trade deals and his strength in a general election campaign.

“I think that over the coming weeks we will join her in that argument,” Obama told reporters. “She has made the argument that she is thoroughly vetted, in contrast to me. I think it’s important to examine that argument.”

His campaign renewed demands that Clinton release tax returns filed with her husband, former President Bill Clinton, since they left the White House. Clinton aides said she would release the returns “on or around April 15.”

Obama, an Illinois senator, pointed to his substantial lead in pledged delegates who will select the nominee at the party’s convention in August, and said he was on the way to the nomination.

Under Democratic rules allowing the losers in each state to win a proportional number of delegates, Clinton must win most of the remaining contests by big margins to have a shot at significantly closing the gap with Obama in the delegate race.

An MSNBC count gave Obama 1,307 pledged delegates to Clinton’s 1,175, with most of the delegate apportionment from Tuesday’s contests still being calculated.

But both candidates are liable to remain short of the 2,025 needed to win the nomination without substantial help from the party’s 796 “superdelegates” -- party officials and insiders who are free to back any candidate.

They could heavily influence the race if they began moving in a bloc toward either Obama or Clinton.

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CLINTON, OBAMA ON SAME TICKET?

The battle also renewed attention on Florida and Michigan, which were stripped of delegates in a dispute with the national party. Clinton won contests in both states, although the candidates did not campaign there, and officials in both states renewed talks about whether the contests should be redone.

Amid the uncertainty, Clinton acknowledged the prospect the two might wind up on the same ticket as presidential and vice presidential candidates.

“Well that may be where this is headed, but we have to decide who is on the top of the ticket,” Clinton, who usually dodges the topic when asked, said on CBS’s “The Early Show.”

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Clinton and Obama both said later it was “premature” to discuss a joint ticket.

Up next are Democratic contests in Wyoming on Saturday and Mississippi on Tuesday, and Obama will be favored in both. The next stop is a big showdown in Pennsylvania on April 22.

“I think that voters are finally focused on who they think would be the best commander in chief and who would be the best president to turn the economy around,” Clinton, 60, said in an interview on NBC’s “Today” show.

Obama, 46, said he was still in control of the Democratic race. “I’m pretty confident that we’re going to end up with more delegates, having won more states, won more primaries, won more caucuses, and have more of the popular vote,” he said.

McCain’s big victories in Ohio, Texas, Rhode Island and Vermont on Tuesday drove his last major rival, former Arkansas Gov. Mike Huckabee, from the race.

“John showed incredible courage and strength of character and perseverance in order to get to this moment,” Bush said in offering his support in a Rose Garden endorsement.

“That’s exactly what we need in a president -- somebody who can handle the tough decisions, somebody who won’t flinch in the face of danger,” said Bush, who defeated McCain in a bitter 2000 battle for the Republican nomination.

Despite Bush’s low approval ratings, McCain said he would welcome the president’s help on the campaign trail.

“And I will be very privileged to have the opportunity being again on the campaign trail with him, only slightly different roles this time.” the former Navy fighter pilot and former prisoner of war in Vietnam added.

(Additional reporting by Ellen Wulfhorst, Caren Bohan and Jeff Mason, Deborah Charles and Donna Smith; Editing by David Storey)