Sen. Evan Bayh of Indiana, who once aspired to the Democratic presidential nomination, on Monday endorsed Hillary Rodham Clinton's bid for the White House.

"I believe she will run a campaign that is both tough and smart when it comes to protecting our nation's security," Bayh said at a news conference with Clinton.

Bayh, a former governor of a Republican-leaning state, is in his second term in the Senate. He won re-election with 62 percent of the vote in 2004 and is considered a possible Democratic vice presidential choice in 2008.

A colleague of Clinton's on the Senate Armed Services Committee, Bayh pursued a possible presidential bid until late last year when he dropped out of the race amid evidence that Clinton and Sen. Barack Obama of Illinois were building formidable campaign organizations.

Clinton said she agreed with Bayh, a centrist, that "Democrats should campaign everywhere in America." He was named a national co-chair of her campaign.

Clinton and Bayh traveled together to Iraq in January, just before Clinton announced she would seek the Democratic nomination. Bayh is one of several potential presidential contenders who have abandoned their own bids to endorse her.

Former Iowa Gov. Tom Vilsack dropped his bid last winter to become the New York senator's campaign chairman. Last week, retired Gen. Wesley Clark also announced he was backing Clinton.

"Hillary Clinton is a seasoned, experienced leader who will be ready to lead this country on Day One," Bayh said.

Earlier this year, Obama suggested that only former President Clinton would be ready from Day One, slighting Sen. Clinton.