By John Whitesides, Political Correspondent
WASHINGTON (Reuters) - Republican John McCain's top two aides left his struggling U.S. presidential campaign on Tuesday, dealing a sharp blow to the Arizona senator and casting doubt on the future of his 2008 bid.
McCain said he would continue his White House run despite the departures of manager Terry Nelson and longtime chief strategist John Weaver, which were announced as McCain took the Senate floor to defend President George W. Bush's strategy in Iraq.
Once a front-runner in the Republican field, McCain has fallen behind as his staunch backing for Bush on the unpopular Iraq war and on an overhaul of immigration laws cost him support among both moderates and conservatives.
"We've had ups and downs in other campaigns and we'll have ups and downs with this campaign. I'm very happy with where we are," McCain told a crush of reporters at the Capitol, promising to "out-campaign" his rivals.
Nelson was Bush's political director during his 2004 re-election campaign and Weaver was McCain's top strategist during his unsuccessful 2000 race for the White House. In statements released by the campaign, neither man offered a reason for leaving.
"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.
Rick Davis, McCain's campaign manager in 2000 and chief executive officer of this campaign, will take over managing McCain's White House run.
FUND-RAISING PROBLEMS Continued...
The top two aides in United States presidential candidate John McCain's campaign have resigned from their posts today.

The resignation of campaign manager Terry Nelson and long-standing chief strategist John Weaver was announced while McCain was on the floor of the Senate debating strategy in Iraq.
John McCain
Nelson said, "It has been a tremendous honor to serve Senator McCain and work on his campaign," and "I believe John McCain is the most experienced and prepared candidate to represent the Republican Party and defeat the Democratic nominee next year."

Weaver's statement echoed that of Nelson: "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."

In a statement, McCain said the resignations were "accepted with regret and deep gratitude for their dedication, hard work and friendship." He further said, "I will continue to address the issues of greatest concern to the American people, laying out my vision for a secure and prosperous America."

Rick Davis will step in as campaign manager. Davis served in this capacity during McCain's 2000 presidential bid.

McCain has not performed well in recent opinion polls. He usually trails Rudy Giuliani, Mitt Romney, and Fred Thompson, pulling only single digits in some surveys.

Last week, McCain announced financial information for his campaign. The US$11.2 million that were raised in the last three months placed him third among Republicans. The $2 million the campaign reported as cash-on-hand was viewed as a relatively small sum by analysts.