Romney told NBC's "Today Show" he believes Obama has failed on the economy, foreign policy and every other area.
"He's been one of the most ineffective presidents that I've ever seen," Romney said.
Asked if he would give Obama a failing grade, Romney responded "absolutely."
Romney, who has a vacation home in New Hampshire, is expected to kick off his presidential campaign in the state Thursday. He generally leads most of the other candidates and potential candidates in what is still a crowded Republican field.
In the interview, Romney continued to blast Obama's healthcare plan while defending his own Massachusetts plan. Obama has said the national plan was modeled on the one Romney created for Massachusetts.
Romney said the economy would be the most important issue and touted his own business career. He said Obama was not to blame for the conditions he confronted when he took office but failed to do anything effective about the slump.
Romney, a Mormon, was also asked if he would have problems with Christian conservatives, who dominate many Republican primaries.
"We're not electing a pastor in chief, we're electing a commander in chief," he answered.