"I'm convinced that the national interest demands a new approach to our interaction with the world," Leach, a foreign service officer before being elected to Congress, said in a conference call with reporters.
Leach predicted that many Republicans and independents would be attracted by Obama's campaign but said his decision to endorse a Democratic presidential candidate for the first time wasn't easy.
"Part of it is political parties are a distant analog to families and you really hate to step outside a family environment," Leach told The Associated Press in an interview.
Leach, 65, was elected to Congress in 1976 and served 30 years before losing a re-election bid in 2006. As a moderate, he was often at odds with the conservative GOP leadership.
"For me, the national interest comes before party concerns, particularly internationally," said Leach, who has long been an opponent of the war in Iraq.
"We do need a new direction in American policy, and Obama has a sense of that," he said. "He recognizes that a long-term occupation of Iraq is not only expensive, it's extremely dangerous to the American interests."
Leach said he was attracted to Obama's call for a dialogue with nations such as Iran that have long been seen as U.S. adversaries.
"He also recognizes that it's preferable to speak with potential adversaries rather than simply shun them," Leach said.
Leach's decision could cause ripples with the group Common Cause, where he serves on the board and which has a policy against political endorsements. Leach said he wasn't aware of any concerns by the nonpartisan group, which supports accountability in government.
Many Republicans argue that GOP candidate John McCain has an edge when the debate turns to foreign policy because of his long experience in dealing with such issues and his record as a career military officer and prisoner of war.
"There's a distinction between trumpeting issues and realistically looking at effectiveness," Leach said. "I have never known a time period where the American brand has been in less repair."
He said most voters instinctively want the U.S. to work with allies, rejecting the go-it-alone approach that has marked the Bush administration.
"If you ask Americans of any political persuasion - conservative, liberal or moderate - whether they'd rather see us lead the world with allies or alone, most people instinctively say we're better off with allies," Leach said. "The public does understand that something is not right about our policies today."