Warner, R-Va., cited McCain's service in the Navy and his familiarity with military issues in a statement backing his longtime friend, who hasn't formally announced his candidacy.
"America's next president will be challenged by a range of diplomatic and security issues of unprecedented complexity largely due to growing, worldwide terrorist threats," Warner said in the statement, released by McCain's presidential exploratory committee. "Senator McCain's long experience with, and understanding of, our military coupled with his proven, unquestioned courage and leadership, provide him with the essential qualifications for our next president."
Warner, a former Navy secretary and until last month the chairman of the Senate Armed Services Committee, has agreed with McCain on a number of national security and defense issues, but differs with him on President Bush's Iraq war policy. McCain backs the president's plan to increase U.S. troops while Warner drafted a bipartisan resolution opposing the buildup.
"America's prosperity and personal freedoms at home must be protected from threats beyond our shores. Security will be the primary issue in the forthcoming presidential campaign," Warner's statement said.
McCain said in his statement that the fifth-term senator's support "means a great deal to me as a person, fellow senator and veteran."
Political analyst Stuart Rothenberg said the timing of Warner's endorsement, despite their differences on Iraq policy, boosts McCain's presidential prospects.
"While Iraq continues to be a trouble spot for the White House and John McCain, here's a senior senator embracing him," Rothenberg said in a telephone interview. "McCain's support on the Hill isn't deep or enthusiastic. To have a senior statesman throwing him his support has got to be a plus for McCain."
McCain, who lost the Republican nomination to Bush in 2000, is expected to formally announce his second campaign for the White House next month.