The campaign manager and chief strategist of Sen. John McCain’s presidential campaign quit Tuesday, dealing a blow to his struggling campaign.

Campaign manager Terry Nelson and chief strategist John Weaver — a longtime adviser to the Arizona Republican — said in separate statements that their resignations would be effective immediately.

McCain vowed his campaign would go on and said of the two aides: “We remain very close friends. We will remain in close contact. ... We will continue to rely on their advice and counsel in the future.”

The Associated Press reported that Rick Davis, a longtime McCain aide, would take over the campaign and other changes also were likely.

McCain, once considered the frontrunner for the Republican nomination, has lost ground steadily over the past few months and now trails former New York Mayor Rudolph Giuliani in most polls. His effort has been hurt by his strong support for the war in Iraq, among other factors.