WASHINGTON — Capping weeks of bad news and sagging support for a one-time Republican frontrunner, the abrupt resignation of Senator McCain’s two top advisers is heightening the perception that his second White House bid is in an inescapable downward spiral.

Mr. McCain’s campaign manager, Terry Nelson, and his senior strategist, John Weaver, announced their departures yesterday morning through a joint statement. The move shocked many in the political establishment but apparently came as little surprise to officials working closely with the campaign, who had anticipated a leadership change when it became clear that the Arizona senator had fallen far short of its fund-raising goals for the second consecutive quarter. The campaign said its chief executive officer, Rick Davis, who ran Mr. McCain’s presidential bid in 2000, would step in as campaign manager.

Other staffers also quit yesterday, including a deputy campaign manager, Reed Galen, and the political director, Robert Jesmer. Another top adviser, Mark Salter, is staying on as an adviser but will not take a salary.

But even as pundits viewed the shake-up as another nail in Mr. McCain’s campaign coffin, the Arizona senator, in an e-mail to supporters, vowed to keep fighting. “I’m determined to continue to face our challenges head-on and win,” he wrote. “The stakes are simply too high to sit on the sidelines during this important crossroad in our nation’s history.”

Mr. McCain added: “Though we have a long, hard road ahead of all of us, I know that with your help, we will prevail.”

At the Capitol, where he delivered a speech on the Senate floor about Iraq, he told reporters that “we’re doing fine” and that he was “happy” with the campaign, according to the Associated Press. Just last week, it was Messrs. Nelson and Weaver who led a conference call to disclose that the campaign was laying off dozens of staffers after finishing the month of June with just $2 million on hand. The total is well below Mr. McCain’s top rivals, Mayor Giuliani and Mitt Romney, and itis less than even a long-shot GOP challenger, Rep. Ron Paul of Texas, has to spend.

Messrs. Nelson and Weaver said at the time that they were confident about the campaign’s future and gave no indication of an imminent change at the top.

Campaign sources said they volunteered their resignations yesterday morning. “They were not fired,” one McCain aide said, speaking on the condition of anonymity.

But there was little doubt that Mr. McCain, frustrated with the slow fund-raising and high rate of spending, was behind the shift.

“The captain of the ship and the future president is making these decisions, and that’s John McCain,” a national finance cochairman of the campaign, Lewis Eisenberg, said in an interview yesterday.

Policy differences did not come into play, sources said. In their statements, Messrs. Weaver and Nelson each reiterated support for Mr. McCain’s candidacy.

Mr. McCain offered high praise for both men in a statement of his own, saying he had accepted their resignations “with regret and deep gratitude for their dedication, hard work, and friendship.” The past six months have seen an increasingly steep decline in Mr. McCain’s fortunes. He entered the Republican presidential nomination race as an early favorite, but Mr. Giuliani soon eclipsed him in national polls. He has acknowledged taking a political hit for his steadfast support for the increasingly unpopular Iraq war. More recently, Mr. McCain’s vocal defense of a failed immigration bill has distanced him from many in the Republican base, causing a drop in the polls and struggles in fund raising.

“He doesn’t have an issue stance that resonates with voters, particularly in the Republican Party,” a political scientist at Hunter College, Andrew Polsky, said.

Mr. McCain’s immediate prospects appear bleak. His is likely to have a hard time generating enthusiasm among fund-raisers watching constant stories about turmoil in his campaign and seeing the simultaneous rise of a former Tennessee senator and almost-certain GOP candidate, Fred Thompson.

“His campaign is on life support,” Mr. Polsky said.

Following the announcement of the leadership changes yesterday, Mr. McCain, Mr. Davis, and other top campaign officials convened an afternoon conference call with national fund-raising chairmen to formally acknowledge the moves and reassure supporters that the campaign would continue.

“It was very upbeat and positive,” Mr. Eisenberg said. He expressed confidence in Mr. Davis as the new campaign manager, saying that the goal going forward, as expressed in the conference call, was to retool the campaign as “a lean, mean fighting machine.”

That description reflects a much slimmer operation than Mr. McCain’s advisers had initially envisioned, focusing on the early primary states and carrying a smaller staff and lower overhead costs. The first test of that strategy will come later this week when Mr. McCain makes his first campaign trip since returning from a visit to Iraq. He is scheduled to deliver a policy address on the war in New Hampshire on Friday.