With the Republican nomination in hand but skeptics remaining on his right, Mitt Romney told a "tax day summit" of tea party activists at the Franklin Institute Monday night that he was determined to shrink the federal government and keep runaway regulation and high taxes from killing jobs.

"The economy is struggling because the government is too big, and we're going to bring it down to size," Romney told about 400 cheering people at the event organized by the Independence Hall Tea Party. "This campaign is going to be fun. The contrast could not be greater."

President Obama "doesn't understand the power and importance of economic freedom," Romney said. "I just don't think he understands what makes America such an exceptional and successful nation."

A wealthy former venture capitalist, Romney also ridiculed the "Buffett Rule," the measure being pushed by Obama and congressional Democrats to ensure the wealthiest Americans pay at least 30 percent of their income in taxes. The proposal was blocked Monday in a Senate vote.

"Someone calculated that the taxes raised from his Buffett Rule would pay for 11 hours of government - and that's his big idea," Romney said of the proposal. "Taxes by their very definition limit our freedom. They should be as small as possible to pay for things that are vital."

Local Democrats took the occasion to demand Romney release more of his tax returns. So far, he has released his 2010 returns and an estimate for 2011 that he will pay an effective tax rate of 13.9 percent because most of his income is from investments.

Monday's gathering was not the type of tea-party rally made famous during the movement's rise in 2009: boisterous groups of fed-up protesters waving yellow Gadsden ("Don't Tread on Me") flags and bringing lawn chairs to the National Mall in Washington. This was a ticketed affair, with $10, $20 and $30 seats; organizers said that paid for the venue and staging.

Indeed, the event drew a different brand of protester - a dozen or so Occupy Philly supporters, who complained of being excluded even though they brandished tickets, and who chanted "refund, refund" on the steps of the building.

Romney's visit with the tea party reflected the delicate task that now confronts him: As the fall race begins in earnest, he must reach out to independent and moderate voters while at the same time reassuring holdouts in the base of his own party.

Exit polls spell out how Romney has faced resistance over the course of the primaries from elements of the Republican coalition - evangelical Christians, including some who are leery of a former Massachusetts governor who also is a Mormon, and voters who identify themselves as "very conservative."

Yet presidential elections are won in suburbs of places like Philadelphia, Cleveland, and Detroit, and along the sprawling I-4 corridor between Orlando and Tampa.

Rick Santorum put an effective end to the Republican nomination race last week when he ended his campaign. Even before that, some tea party groups and officeholders allied with the movement had embraced Romney.

A few days after Santorum's exit, Romney won endorsements from three social conservative organizations - the National Organization for Marriage, National Right to Life and the Susan B. Anthony List, a pro-life group.

"Along the way we reserve the right to encoruage our party nominee to more closely align with our values," said Michael Leahy, leader of a group called Tea Party Nation, who introduced Romney Monday. "But let me tell you, we're going to be out there campaigning hard for him."

Lucy Brown, 88, a tea party sympathizer from Old City who attended, said the need to beat Obama made her enthusiastic about Romney. "I think the tea party has been going overboard in saying he's not conservative enough," said the retired nurse. "No one person is perfect, but he's not a socialist, he's not a Marxist. He's not any of those things."

Sandy Niemotka, 45, a lawyer from Oaklyn in Camden County, is a tea party member but also a Romney fan, liking his experience in business and as governor.

"He gets such a bad rap, but I think he's great," said Niemotka. "He's being made fun of on Saturday Night Live for being a nerd and maybe a little bit too much of a salesman, but at least our guy for once isn't being made fun of for being dumb."

Other agreed: The candidate need not be perfect.

"One of the things I remind people is to focus on issues, and not on the desire to have Ronald Reagan re-incarnated," said Jennifer Stefano, a tea party leader in Bucks County, said in a phone interview. "No candidate is going to be that perfect ideal. The question is, can we as a nation take another four years of economic hell unleashed on us by our president?"

She predicted most tea party activists would rally to Romney, noting that Obama is not responsive to the movement's philosophy of limited government while the "whole trajetory" of Romney's career is "moving in that direction."