Massachusetts Senator John Kerry praised President Obama's foreign policy accomplishments during his speech at the Democratic convention Thursday night and, in a bit of self-deprecating humor, labeled Mitt Romney a flip flopper using the same phrases Kerry's opponents used against him eight years ago.

"It isn't fair to say Mitt Romney doesn't have a position on Afghanistan," Kerry said. "He has every position."

Discussing the Libyan intervention, Kerry said Romney had adopted several conflicting opinions, an indication that he'd be a weak leader on foreign policy.

"Talk about being for it before you were against it," Kerry said, a reference to the line Kerry himself infamously used to describe his record on the Iraq War. "Mr. Romney, here's a little advice: Before you debate Barack Obama on foreign policy, you better finish the debate with yourself."

Even on more benign foreign policy duties, Kerry said Romney was not fit to lead. Citing Romney's widely-panned overseas trip, in which the Republican presidential nominee drew criticism from local leaders in Britain and elsewhere, Kerry said Romney did not know how to represent America well.

"For Mitt Romney, an overseas trip is what you call it when you trip all over yourself overseas. It wasn't a goodwill mission—it was a blooper reel," he said.

Kerry said Obama, by contrast, had proven time and again that he was a capable world leader. Answering the question du jour of late—whether Americans are better off now than they were four years ago—Kerry added a twist to answer that question in terms of national security.

"Ask Osama bin Laden if he is better off now than he was four years ago," he said.

Raising the same themes central to his 2004 presidential campaign, Kerry, the chairman of the Senate Foreign Relations Committee, said Obama's efforts had restored America's standing on the world stage by repairing relationships frayed by his White House predecessor, George W. Bush.

"It took President Obama to make America lead like America again," Kerry said. "It took President Obama to restore our moral authority—and to ban torture. This president understands that our values do not limit our power—they magnify it. He showed that global leadership is a strategic imperative for America, not a favor we do for other countries."

Watch the whole speech below: