President Barack Obama's campaign formally welcomed Republican Mitt Romney to the White House duel Tuesday, with a caustic warning: the more Americans see of him, the less they like him.

Romney effectively clinched the Republican presidential nomination when his last remaining rival Rick Santorum bowed out, setting up a nasty, battle with Obama, who will ask voters for a second four-year term in November.

The president and the former Massachusetts governor have actually been squaring off for several weeks, but Obama's team could not pass up another chance to try to negatively define Romney in the eyes of voters.

"The more the American people see of Mitt Romney, the less they like him and the less they trust him," Obama campaign manager Jim Messina said in a statement issued shortly after Santorum suspended his campaign.

Messina accused Romney of alienating key voting blocs including women, the middle class, and Hispanics and framed the election as a fight for a fair economy, a theme Obama spent the day hitting in swing state Florida.

The president touted his millionaires tax in Florida, a battleground state which could play a decisive role in the November 6 election.

The initiative, calling for a minimum tax of 30 percent on those earning more than $1 million a year has no chance of quickly becoming law, but it anchors Obama's vow to forge an economy where everyone has "a fair shot."

"What drags our entire economy down is when the benefits of economic growth and productivity go only to the few," Obama said.

"The gap between those at the very, very top and everybody else keeps growing wider and wider and wider and wider," Obama said.

The president argued that government-led investments in the future economy were not a "socialist dream" as some of his conservative opponents would have it, but were essential to future prosperity.

"Let me you ask you: what's the better way to make our economy stronger? Do we give another $150,000 tax break to every millionaire and billionaire in the country?" the president said.

"Or should we make investments in education and research and health care and our veterans?"

Obama calls his plan the Buffett rule, after billionaire financier Warren Buffett who complained that his massive investment income was taxed at a lower rate than the taxes his secretary pays on her wages.

Congress was expected to vote on the millionaires tax next week and the plan has almost no chance of passing.

But Democrats want to force Republicans to cast a vote to oppose the tax, which Obama and party allies can then use to castigate their foes on the campaign trail ahead of November's election.