Republican presidential contenders have hammered President Obama this week as a cautious and ineffectual world leader, insisting that he lacks — in the words of Michele Bachmann — a “ titanium spine.”

But Vice President Joe Biden proved Tuesday he won’t let the attacks go unanswered, telling a crowd of campaign donors in Tulsa, Okla., that the president has “a backbone like a ramrod.”

“Presidential elections are about strength and character,” Biden said, according to press pool reports. “It’s the one race that’s measured different than any other in American politics. It comes down, in relative terms, to has the most strength and the most character.

“People knew Barack was really bright, they knew Barack was straight, they knew Barack was a different kind of politician. What they didn’t know was just how strong he was,” he said. “Republicans spent a lot of time trying to tag him as a follower, not a leader. Well, he’s put that to bed.”

Biden cited Obama’s gutsy decision to order a covert raid on Osama bin Laden’s Pakistani compound as proof the president is a strong commander in chief. And he said Obama’s role in the debt ceiling debate was equally strong, brokering a deal with Republican leadership that ultimately fell to Tea Party intransigence.

“This is going to be the clearest-cut election,” Biden said. “This is going to be the clearest choice of where we want to be. These guys have laid it out where they want us to be. And we’re going to debate it. We’re not going to debate just in terms of, you know, a 30-second political ad. Paul Ryan laid out their budget. Their budget, over the next decade, eviscerates — eliminates — Medicare. They say it’s a voucher system. I call that eliminating it.”

Biden made the remarks at a private fundraiser at the Tulsa home of Dr. Stephen Adelson. Donations benefitted the Obama Victory Fund, a joint account that funnels money to Obama’s reelection campaign and the Democratic National Committee.