WASHINGTON — With Republican John McCain edging ahead of Democrat Barack Obama in the latest opinion polls, the two candidates are now locked in bitter political fight over a core issue: who can best claim the mantle of change.

Obama, who founded his campaign on a pledge to reform Washington, yesterday unleashed new TV advertisements, revised his stump speech and released a strategy memo, all of which challenge McCain's efforts to cast himself as a maverick and reformer who can bring change to Washington.

McCain vowed in a TV interview to appoint Democrats and independents to his administration if elected. McCain's campaign also unveiled an ad that promises "Change is coming," the cry McCain has adopted since he accepted his party's nomination on Sept. 4.

The latest back-and-forth highlighted that both campaigns believe that whoever can make the best case for changing Washington will win the White House. Rather than the economy, Iraq or other specific issues, "change" has become the most heated subject of debate as the race enters its final seven weeks.

McCain's shift has been most dramatic since he chose Sarah Palin, the little-known governor of Alaska, as his running mate two weeks ago. Since then, he has largely abandoned his long-standing claim that his 26 years of experience in Congress was his chief qualification for the Oval Office.

Instead, McCain is breaking new ground in presidential politics by essentially bashing his own party, as well as his opponent, as he casts himself as someone who can change the way Washington does business.

Experts say no candidate in memory has worked so hard to disassociate himself from the party he now leads as presidential standard-bearer. In speeches and printed handouts, McCain rarely even identifies himself as a Republican.

McCain's advertising goes further. A television ad launched last week in Ohio and other battleground states suggests that McCain and Palin are at war with their own party.

"He battled Republicans and reformed Washington," the announcer intones. "She battled Republicans and reformed Alaska."

In his new TV ads and in campaign events yesterday, Obama struck back by insisting McCain represents "more of the same" as President Bush, and he ridiculed McCain's claims that he is not a traditional Republican.

"John McCain's economic policies are identical to George Bush's. His tax policies are identical to George Bush's. His education policy, which is essentially nothing except arguing for vouchers — identical to George Bush's," Obama, a first-term senator from Illinois, told a rally of about 1,500 people in Concord, N.H.

"Change has to be more than a slogan," Obama says to the camera in one new television ad. Obama's aides said privately that they will try to ignore Palin, a public sensation who has helped reinvigorate McCain's campaign, and that they expect her high profile to diminish as her novelty wears off. They hope to refocus voters' attention on McCain's record of voting to support Bush administration policies.

For now, McCain's recasting himself as the agent of change appears to be paying off. According to the latest Gallup survey, 54 percent of Americans believe McCain would be effective in changing the way things are done in Washington — close to the 61 percent who believe this about Obama.

McCain "has been saddled with the scarlet letter 'R,' " for Republican, said Larry Sabato, director of the Center for Politics at the University of Virginia. "He's trying to change the scarlet letter to an 'I.' He's trying to run as an independent."

McCain has built a record of sometimes criticizing his own party and reaching out to Democrats on such issues as immigration and campaign finance. And candidates for national office often lambaste Washington even as they seek to move or extend their stay there.

But McCain's fervent attempt to carve a new public image goes deeper, analysts say.

When he accepted the Republican presidential nomination in St. Paul, he avoided uttering the name of the current, unpopular White House occupant. But Bush got off easy compared with the Republican Party.

"We lost the trust of the American people when some Republicans gave in to the temptation of corruption," McCain declared.

Some GOP veterans warn that McCain risks going too far and alienating the grass-roots Republicans whom he needs to lick envelopes, knock on doors and get out the vote on Election Day.

"He's walking a tightrope there," said Stuart Spencer, a former adviser to President Reagan, citing McCain's party-bashing convention speech. But he added, "Part of the Democratic position is they're going to tie him to Bush, and he's got to untie himself. This is one way of doing it."