MIAMI - US President Barack Obama on Monday lashed back at critics who fear he lacks the steel to be a successful president, saying "I'm skinny ... but I'm tough."

Obama, who is trying to navigate a climate of sharp political partisanship, said he was spoiling for a fight to enact his ambitious agenda.

"There are people saying mean things about me and folks are worried," Obama said, as he headlined a pair of Florida Democratic Party fundraisers that racked up a total of 1.5 million dollars.

"Just 'cause I'm skinny doesn't mean I'm not tough. I don't rattle," Obama said, tongue in cheek.

"I'm not going to shrink back, because now is the time for us to continue to push and follow through on those things that we know have to be done but haven't been done for decades."

Obama reeled off a list of what he billed as the early achievements of his administration, including rescuing the economy from a possible depression.

He also said he was "confident" that he would be able to force a landmark healthcare reform bill through Congress by the end of the year.

A number of commentators have in recent weeks started to question Obama's fortitude, worrying that his cool demeanor masks a lacking killer instinct.