Former President Bill Clinton will today break the informal 9/11 Anniversary truce between the rival presidential campaigns, holding a rally for President Obama on a university campus in south Florida.

At the event - the only political rally of the day - Clinton will "discuss the choice in this election," the Obama campaign said. It's a discussion likely to include criticism of Obama's GOP rival, Mitt Romney, in an effort to highlight contrasting visions.

Some Republicans are calling the Clinton appearance unseemly on a day when politicians on both sides of the aisle - including Obama, Romney, Vice President Joe Biden, and Rep. Paul Ryan - have struck a somber and nonpartisan tone in official remarks and appearances.

None of the candidates have engaged in overt electioneering and their campaigns have pulled TV ads off the air.

"The true legacy of 9/11 will not be one of fear or hate or division. It will be a safer world; a stronger nation; and a people more united than ever before," Obama said at the Pentagon memorial service.

"With less than two months to go before Election Day, I would normally speak to a gathering like this about the differences between me and my opponent's plans for our military and for our national security," Romney later told the National Guard Convention in Reno. "There is a time and a place for that, but this day is not it."

An Obama campaign official said Clinton's appearance was not inappropriate and that he would show deference to the anniversary in the tone and substance of his remarks.

"While in Miami today, President Clinton will honor the anniversary of September 11th by observing a moment of silence at his evening event," the official said. "He will also be introduced by a local firefighter/paramedic who assisted in the recovery efforts after Hurricane Katrina and the earthquake in Haiti.

"I don't anticipate heavy contrast today," the official added, referring to Clinton's treatment of Romney.

A spokesman for the Romney campaign declined to comment on the Clinton event.

Campaigning on the 9/11 anniversary is not unprecedented. Sen. John McCain held a rally in Sioux City, Iowa, on Sept. 11, 2007.