Rick Davis, John McCain's presidential campaign manager from 2000, has been tapped to revitalize the Arizona's senator's flagging 2008 bid, McCain's campaign announced Tuesday.

Davis, who was previously chief executive officer, is filling the post left vacant when campaign manager Terry Nelson resigned earlier in the day.

"This campaign has always been about John McCain and his vision for reducing federal spending, defending traditional values and winning the war against Islamic extremists. Today we are moving forward with John's optimistic vision for our country's future," Davis said.

Nelson wasn't the only one whose departure shook up the campaign, which is currently suffering from a dearth in donors and dropping poll numbers. Chief strategist John Weaver, a longtime McCain aide, also tendered his resignation.

McCain said he accepted the resignations of Nelson and Weaver "with regret and deep gratitude for their dedication, hard work and friendship." He also denied that he had asked for the two resignations as part of an effort to inject energy into the campaign.

"In the days and weeks ahead this campaign will move forward, and I will continue to address the issues of greatest concern to the American people, laying out my vision for a secure and prosperous America," he said in a written release.

"The campaign intends to move forward," an official close to the campaign told FOX News.

In written statements released Tuesday, the two men who had been working without salaries, said they were honored to have worked with McCain, who had been the presumptive Republican frontrunner ahead of the election season.

"This morning I informed Senator McCain that I would be resigning from his presidential campaign, effective immediately. It has been a tremendous honor to serve Senator McCain and work on his campaign. I believe John McCain is the most experienced and prepared candidate to represent the Republican Party and defeat the Democratic nominee next year," Nelson said.

"It has been my honor and a distinct privilege to serve someone who has always put our country first. I believe that most Americans will come to the conclusion that I have long known there is only one person equipped to serve as our nation's chief executive and deal with the challenges we face, and that person is John McCain," said Weaver, who has been with McCain for 10 years.

Officials say Nelson and Weaver stepped down because of what they call the "well-documented" challenges of the campaign. A former McCain staffer from the 2000 campaign calls Weaver's exit an "earthquake" and inconceivable without McCain's approval.

Whether Weaver was fired or not appears less significant than the fracturing of a political relationship years in the making and one that had few equals in American politics in terms of trust and mutual respect.

"It would be like President Bush without Karl Rove," the former McCain 2000 staffer said.

Sources say it became obvious that with only $2 million in cash on hand, a pittance compared to his rivals, McCain's staff needed a dramatic change. Already, the second-quarter fundraising numbers, totaling $11.2 million for the time frame from April through June, brought a second wave of layoffs following a poorer-than-expected showing in the first quarter.

But the biggest problem for the McCain campaign has been its inability to control campaign costs. As many as 50 staffers, including many in McCain's Iowa organization, now have been let go from the campaign. Others have taken salary cuts and staff have been reshuffled in New Hampshire, South Carolina and elsewhere.

Despite McCain's playing down the turnover, an official familiar with campaign operations tod FOX News the resignations most definitely could be defined as a shake-up.

"You cannot drop 50 or so staffers and not lose the guys at the top making the decisions. Terry already said last week he would work for free. This is not about money. This is about needing a change in strategy," the official said.

Mark Salter, a former chief of staff in McCain's Senate office, whom some consider McCain's alter ego, will continue to advise him in a voluntary role but will cease his day-to-day involvement in the campaign.

One official characterized Salter as "someone integral to the message and how it is formalized" on the campaign trail.

McCain, who just returned from his sixth trip to Iraq, will be discussing his visit in a major speech in New Hampshire on Friday. On the Senate floor Tuesday, he defended the troop buildup, which has only been complete for a few weeks.

"Make no mistake. Violence in Baghdad remains at unacceptably high levels," but the United States and Iraq seem to be "moving in the right direction," McCain said. "The progress our military has made should encourage us."

FOX News' Carl Cameron, Major Garrett and Trish Turner contributed to this report.