ABC News' Michael Falcone and Shushannah Walshe report:

Ohio Attorney General Mike DeWine has had a change of heart.

The prominent Buckeye State Republican threw his support behind presidential candidate Rick Santorum on Friday afternoon, switching his allegiance from Mitt Romney to the former Pennsylvania senator.

"To be elected President, you have to do more than tear down your opponents," DeWine said in a statement.  "You have to give the American people a reason to vote for you - a reason to hope - a reason to believe that under your leadership, America will be better.  Rick Santorum has done that.  Sadly, Governor Romney has not."

DeWine made the official the announcement on Friday in front of the Ohio State House in Columbus. There he candidly admitted he "was wrong" to have endorsed Mitt Romney four months ago and he was even more candid about why.

According to DeWine, Romney has shown an "astounding inability to provide voters with a rationale to support him."

As for Santorum, "People like him - he's human!" DeWine said.

He cited Romney's "massive financial and organizational advantage" as one of the reasons he previously supported the former Massachusetts governor's presidential bid.

Evidently pleased by DeWine's scathing critique of Romney, Santorum said he only wished his wife could have been in Ohio to attend the endorsement event: "I wish Karen could have heard that," he said.

Santorum will become the third presidential candidate DeWine backed during the primary season. The former U.S. Senator who served alongside Santorum in Washington, originally endorsed Tim Pawlenty before moving to Romney after Pawlenty dropped out of the race.

In a tweet, Romney strategist Eric Fehrnstrom sought to portray DeWine as fickle: "Mike DeWine makes his third endorsement in six months - Pawlenty, Romney, and now Santorum. I think that's a record," Fehrnstrom wrote.

Romney is not lacking in Ohio endorsements. He has dozens of state and local elected officials on his side, including U.S. Sen. Rob Portman. But on Friday he lost one of them.

"For some time now, it has been clear to me that Rick Santorum should be the Republican nominee for President," DeWine said in the statement circulated by the Santorum campaign. "To be frank, I've had some sleepless nights.  I could not, in good conscience, be on record endorsing Governor Romney when I knew in my heart that Rick Santorum was the better candidate."

Earlier on Friday, Santorum held a town hall meeting in Shelby Township, Mich. and dropped DeWine's name when a voter asked Santorum about his 18-point loss in his U.S. Senate re-election campaign in 2006.

"It was a horrible year. We lost 23 of 33 senate races in this country. I mean it's one thing to say oh you lost your last race, why could you run? But everybody lost the race in 2006.," Santorum said. "So if you Look here at Michigan. Look who you re-elected in 2006. Let's just get down to the fact of how bad it was. Mike DeWine in Ohio lost by 15 points, in the state of Ohio, which is a better state than Pennsylvania. And by the way he's now the attorney general of Ohio. He came back four years ago and he won."

DeWine served for two terms in the U.S. Senate and four in the House of Representatives. He announced his endorsement of Romney in last October.

"Americans can choose a president that believes in the greatness of America or can choose a president that would rather engage in class warfare and partisan bickering than work to solve America's problems," DeWine said of Romney back then in a statement posted on his Facebook page.