WASHINGTON — John McCain jettisoned his two top aides Tuesday as the one-time Republican front-runner struggled to right a presidential bid in deep financial and political trouble.

Campaign manager Terry Nelson and chief strategist John Weaver offered McCain their resignations, which the Arizona senator accepted with "regret and deep gratitude for their dedication, hard work and friendship."

At least three other senior aides followed the two out the door, and the campaign announced that Rick Davis, who managed McCain's 2000 bid and has served as the current campaign's chief executive officer, will take over.

"I'm determined to continue to face our challenges head-on and win," McCain said in an e-mail to supporters, vowing to press on. Aides insisted he would not drop out of the race.

The second major staff shake-up comes after behind-the-scenes maneuvering among senior advisers for control of the campaign and as McCain grapples with several problems ranging from his dwindling bank account of some $2 million to slippage in opinion polls. He also has staked out politically unpopular positions on two key issues — the Iraq war and immigration — that have hindered his candidacy.

Considered the leading GOP candidate as the year began, McCain now faces significant hurdles to winning the Republican nomination that eluded him seven years ago. The senator is essentially restarting his campaign six months before the first voting begins.

"I think we're doing fine. I'm very happy with the campaign the way it is," McCain said at the Capitol, even as the departures roiled his staff. He spoke to reporters after delivering a speech in the Senate in which he reiterated his defense of President Bush's troop increase in Iraq.

His backers long have argued that, in the end, GOP primary voters will gravitate toward the 70-year-old's record of experience, leadership and character when they survey the entire GOP field. The stakes are even higher now, given the few options left to revive his candidacy.

McCain's situation resembles that of John Kerry, who struggled in fund-raising, languished in polls and fired top aides in late 2003. A few months later, the Massachusetts senator won the Democratic nomination. He lost to Bush in the general election.

"John McCain's appeal as a stalwart defender of his principles and as an American hero continues to give him a seat at the table regardless of his financial standing," said Whit Ayres, a Republican pollster who is unaligned in the race. However, he added: "it becomes very difficult to run an effective campaign anywhere without significant resources."