Sen. John McCain (R-Ariz.), who has been among the staunchest supporters of President Bush’s effort to increase troop levels in Iraq, is set to receive the endorsement of one of the most prominent critics of the “surge” plan.

Sen. John Warner (R-Va.) will announce his support of McCain’s 2008 presidential bid today, lending the name of one of the chamber’s most respected old bulls to the Republican frontrunner’s developing Capitol Hill support.

In a brief interview, Warner, a former Armed Services chairman and one-time secretary of the Navy, cited his long personal history with McCain — as well as the importance of national security issues in the 2008 campaign — as the chief factors that led to his endorsement.

“To me, voters are going to seek out that candidate, be it Republican or Democrat, that’s got the proven credentials on issues that concern security abroad and here at home,” Warner said.

McCain, a Naval aviator who spent five and a half years as a prisoner of war in Vietnam, was released from captivity in 1973 while Warner was serving as Navy Secretary.

Warner recalled that on his trips to Vietnam as secretary, he would stop in Hawaii to be briefed by Adm. John McCain Sr., Sen. McCain’s father who was commander in chief of the U.S. Pacific Command at the time.

“Of the people who worked with me in that period … he was one of the most valued advisers that I had in that intense period of the war,” Warner said of the senior McCain. “He was absolutely superb.”

In recent weeks, however, Warner has been the chief GOP spokesman in the Senate against Bush’s latest Iraq War strategy, going so far as to draft a bipartisan resolution that expressed that opposition.

McCain and his two main rivals for the 2008 GOP nod, former New York City Mayor Rudy Giuliani and former Massachusetts Gov. Mitt Romney, support the president’s plan to increase troop levels in Iraq by 21,500.

But while Warner and McCain have been on opposite ends of the surge debate, the two men have a long history of shared causes.

Warner noted that his “fierce battles” against McCain on the Senate floor only served to boost their mutual respect.

“We’ve gone toe-to-toe together and we’ve gone arm-and-arm together,” Warner said. “I think that strengthens the relationship.”

In the previous Congress, McCain and Warner were instrumental members of the “Gang of 14,” which sought to avert a Senate showdown over judicial nominations.

Also, Warner and McCain backed now-Senate Minority Whip Trent Lott’s (R-Miss.) leadership bid last fall. Lott is now one of McCain’s most active presidential backers.

Last week, McCain’s campaign rolled out the endorsements of Sen. John Thune (R-S.D.) and Rep. Chip Pickering (R-Miss.), both of whom are close to Lott and are far more conservative than Warner.

While political insiders debate the ultimate worth and impact of Congressional endorsements, McCain’s campaign has been steadily adding to its ranks after getting off to what was perceived as a slow start.

Romney was most aggressive out of the gate in January in garnering Hill supporters and some Republican observers have privately speculated that McCain’s team was caught off-guard initially.

With the addition of Warner, McCain now has eight backers in the Senate and 17 in the House, with more to come soon. Both Romney’s and Giuliani’s support is rooted almost entirely in the House.

In a recent interview, McCain acknowledged he has been slow to add Congressional endorsements, saying he’s been preoccupied by the Senate debate over Bush’s troop proposal. McCain took a leading role in the discussions as one of the top supporters of the plan.

“I am working on it,” McCain said of lining up backers. “I’ve been working so much on Iraq, I haven’t had as much time meeting with people. I’ll take some time when the Iraq debate dies down a little.”

He also said that Congressional endorsements were less about numbers and more about the weight those supporters carry back home.

While Giuliani has been out front in most public polls that tested the GOP field, McCain’s national security credentials, political apparatus and leftover support from his unsuccessful 2000 bid put him first among most political handicappers in the race for the 2008 nod. Still, McCain’s thorny relationship with social conservatives remains the biggest wild card in his path to the nomination.

Romney, who is the least-known of all three top contenders, began a TV ad campaign last week to boost his profile.

Meanwhile, McCain also has recently announced the support of several people with ties to Capitol Hill.

Former Sen. Mike DeWine (R-Ohio), who was defeated for re-election last year, will chair McCain’s campaign in Ohio and former Sen. Warren Rudman (R-N.H.) will be a co-chairman of his national exploratory committee. DeWine was a member of the Gang of 14 along with McCain.

Last week, McCain also got the backing of Indiana Gov. Mitch Daniels (R), a former Office of Management and Budget director, and moderate Rep. Mike Castle (R-Del.).

Erin P. Billings contributed to this report.