President Obama has rarely engaged Mitt Romney directly over policy and politics so far during the presidential primary campaign.

But today Obama didn't pass up an opportunity when asked about GOP frontrunner's claims, made over the weekend, that the president does not believe in the "exceptionalism" of the U.S.

"It's worth noting that I first arrived on the national stage with a speech at the Democratic Convention that was entirely about American exceptionalism and that my entire career has been a testimony to American exceptionalism," Obama said at an afternoon Rose Garden press conference.

"But, you know, I will cut folks some slack for now because they're still trying to get their nomination," he added, without mentioning Romney by name.

During a Saturday speech in Pewaukee, Wisc., Romney questioned Obama's commitment to the view of America as a unique and unrivaled world power sustained by the values of free enterprise.

"Our president doesn't have the same feelings about American exceptionalism that we do," Romney said Saturday. "And I think over the last three or four years, some people around the world have begun to question that. On this Tuesday, we have an opportunity - you have an opportunity - to vote, and take the next step in bringing back that special nature of being American."

Wisconsin, Maryland and the District of Columbia all hold primary votes on Tuesday.

Canadian Prime Minister Stephen Harper and Mexican President Felipe Calderon, who joined Obama in the Rose Garden on Monday, declined to respond to Romney's claim that America's influence has declined under Obama.

But Harper praised the president and the U.S. for its leadership over the past three years.

"For Canada, the United States is and always will be our closest neighbor, our greatest ally and our best friend.  And I believe that American leadership is at all times great and indispensable for the world," Harper said.

"We had under your leadership, Barack, that successful intervention in Libya.  Our trade relationship is the biggest in the world and growing.  And so I think it's been a tremendous partnership," he said.