Republican Sen. John McCain will lay out his case for the presidency Wednesday, claiming he has the experience to lead a nation at war as he seeks momentum for his troubled campaign.

"I know how to fight and how to make peace. I know who I am and what I want to do," McCain says in a speech marking the official beginning of his second White House bid seven years after losing the GOP nomination to George W. Bush.

"We face formidable challenges, but I'm not afraid of them. I'm prepared for them," the four-term Arizona senator, ex-Navy pilot and former Vietnam captive says in speech excerpts his aides made available.

Seeking to turn a potential liability into an asset, the 70-year-old who could be the oldest first-term president adds: "I'm not the youngest candidate. But I am the most experienced."

His speech, and a several-day "announcement tour" through early primary states, is a formality. McCain has been building a national campaign organization for years and has been campaigning in Iowa, New Hampshire, South Carolina and elsewhere for months.

The high-profile events do, however, give McCain an opportunity to restart his White House bid.

Once considered the front-runner to replace Bush, McCain cast himself as Bush's inevitable successor as 2006 ended, but his campaign has gradually faltered since then. He now finds himself trailing former New York City Mayor Rudy Giuliani in national polls and ex-Gov. Mitt Romney in fundraising and money on hand.

Early in the year, McCain became linked to the president's policy to increase the number of troops serving in the Iraq war — and he became the top pitchman for it in Congress as support for the conflict continued to wane. He has staked his candidacy to the war's outcome.

In recent weeks, McCain has made a few verbal gaffes, trimmed staff positions, instituted spending controls and revamped his fundraising operation. On Tuesday, aides said he replaced his fundraising chief, Carla Eudy, with Mary Kate Johnson, who was deputy finance director for the Bush-Cheney campaign in 2000. Eudy was named a senior adviser to the finance team.

To breathe new life into his campaign and lay out his vision for the country, McCain chose to return to the state of his 18-percentage-point upset over Bush in the 2000 primaries when the senator was a plucky insurgent seeking to knock off the Texas governor who had the backing of the GOP establishment. Bush beat McCain in South Carolina in a bitter race and his campaign never recovered.

Now, McCain is seeking the support of the very Republican core he once spurned at nearly every turn — and is claiming he's the most qualified person to be president as the country wages war in Iraq and faces threats from terrorists.

"I know how the military works, what it can do, what it can do better, and what it should not do. I know how Congress works, and how to make it work for the country and not just the re-election of its members," McCain says. "I know how the world works. I know the good and the evil in it. I know how to work with leaders who share our dreams of a freer, safer and more prosperous world, and how to stand up to those who don't."