Senate supporters of Sen. John McCain’s presidential bid remained publicly optimistic about the Arizona Republican’s ability to get back on track, even after a major senior staff shake-up Tuesday produced palpable unease about the continued viability of his 2008 campaign.

Campaign manager Terry Nelson and John Weaver, McCain’s longtime top political strategist, announced their departures from the campaign. Rick Davis, another longtime McCain adviser and the manager of his failed 2000 presidential primary bid, will step in to fill the leadership void.

Deputy campaign manager Reed Galen and political director Rob Jesmer also resigned their positions with McCain’s campaign Tuesday. Mark Salter, another longtime senior aide who is very close to the Senator, will continue to serve as a senior adviser to the campaign but on a pro bono basis.

The staff shake-up is only the latest evidence to underscore the tenuous status of the one-time GOP frontrunner’s 2008 effort.

McCain’s campaign ended June with about $2 million in the bank — after raising about $24 million total — and last week he announced staff layoffs and salary cuts to help ease the wide financial gap with other leading presidential contenders.

Still, McCain loyalists insisted Tuesday that the maverick Senator whose popularity soared aboard the “Straight Talk Express” in 2000 hasn’t reached the end of the road yet.

“This is far from obituary time,” said Sen. Richard Burr (R-N.C.), one of the nine Senate colleagues who have endorsed McCain’s White House bid.

He added: “I don’t think John’s support has changed at all up here.”

Burr said that despite fluctuating polls, McCain remains the best-known entity among the GOP field and that fact above all others should not be discounted.

He also argued that McCain’s $11.2 million second-quarter haul (less than his first-quarter take) was only about $2 million short of what former Massachusetts Gov. Mitt Romney and ex-New York Mayor Rudy Giuliani, two of the GOP frontrunners, raised from supporters.

“He’s right in the ballgame from the standpoint of what he’s been able to raise,” Burr said.

Sen. Olympia Snowe (R-Maine), however, was somewhat less certain about McCain’s continued viability, though she said he is doing the right thing in revamping his campaign strategy now.

“It remains to be seen,” Snowe said. “Obviously he’s taking the necessary and essential steps.”

She added that while it is still early in some ways, “It is important for him to figure out the strategy for the future.”

Shortly after news of the staff shake-up broke Tuesday, McCain went to the Senate floor to reiterate his support for continued military efforts in Iraq — an unpopular position that, along with McCain’s stance on immigration reform, has caused his campaign consistent heartburn.

A few hours later McCain patiently took questions from a throng of reporters after the Senate’s weekly policy luncheons — even pausing at one point to take a cell phone call in the midst of the crush.

He dismissed the notion that he might not compete in the Iowa caucuses and described the changes as part of routine “challenges” and “ups and downs” of a campaign.

“The campaign will continue,” he said.

Snowe said she had not talked to McCain about the campaign overhaul and there were no immediate indications that McCain had reached out to his supporters on the Hill to reassure them of his ability to recapture momentum, though he did alert his financial backers in a conference call.

Sen. Lindsey Graham (R-S.C.), McCain’s closest Senate ally, described the shake-up as “a realignment” designed to reinvigorate the campaign and assign staff to more appropriate positions.

Graham downplayed the significance of the staff changes, saying what’s important is McCain’s vision for the country, and his courage to take on the tough issues from immigration reform to the Iraq War.

“I think the campaign needed to re-evaluate where it was at,” Graham said. “[McCain] wanted to put people in places and in positions that will highlight their strengths.”

“I think that’s what this is all about,” he added.

Privately, knowledgeable sources said the rift over strategy between Nelson and Davis, the campaign’s chief executive officer, had been evident for some time. Nelson initially had offered his resignation after the second-quarter fundraising numbers became public, but McCain would not take it.

Nelson was political director for President Bush’s 2004 re-election campaign and it was viewed as a major coup when McCain hired the Bush insider.

Weaver, who also served as a top adviser to McCain’s 2000 campaign, praised Nelson as “the finest campaign manager” in a statement to campaign staff and leadership.

“The Senator today decided to move the campaign in a different direction,” Weaver wrote. “After making that decision, the Senator asked me to stay in my current position, but given the fundamental shift, I could not do so. Therefore I resigned this morning.”

The statement also said McCain will always have Weaver’s support.

In addition to his work for McCain, Davis served as deputy campaign manager for former Sen. Bob Dole’s (R-Kan.) 1996 presidential campaign.

“I hope you will join me in welcoming back an old friend, Rick Davis, who will be resuming his role from the 2000 campaign as my campaign manager,” McCain wrote in an e-mail to supporters Tuesday afternoon. “Though we have a long, hard road ahead of all of us, I know that with your help, we will prevail.”

Erin P. Billings contributed to this report.