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.

The so-called cinnamon challenge—a dare to swallow a spoonful of cinnamon without water—has gone viral and beyond. Though the challenge has been around for years, its popularity has spiked recently, to the amusement—or puzzlement—of many.

Some 30,000 videos tagged "cinnamon challenge" have been uploaded on to YouTube. The most popular, with almost nine million views, was uploaded last month; it shows a woman with big earrings slurping a pile of brown powder from a soup ladle and immediately, dramatically, spitting it out. A fit of coughing follows.

The difficulty is that the spice doesn't break down very fast and can get stuck in the throat, causing gagging and even vomiting. Doctors say this can be dangerous because the cinnamon can prevent air from reaching the lungs. "It is an obvious choking hazard and there is a risk of inhaling the dust. This certainly is not advisable," says a spokesman from the Food and Drug Administration.

Dr. Jeffrey Cain, president-elect for the American Academy of Family Physicians, says the cinnamon itself isn't dangerous—but inflammation of the lungs is a real possibility. That, and being laughed at, he says.

As a result, schools from Alabama to Guam are warning parents and staff about the potential dangers of swallowing so much cinnamon at once. "The kids all know about this from the Internet but the parents have no idea," says Arthur Williams, principal at Huron High School in Ann Arbor, Mich., who emailed parents after a student was recently hospitalized for 4½ days because of lung trouble after trying the challenge.

Pottstown Middle School in Pottstown, Pa., which has had three reported incidents of students taking the cinnamon challenge on campus since January, caught a student trying to smuggle a vial of cinnamon into school in a pair of boots. The school subsequently put a ban "open-top boots," also intended to stop cellphone smuggling.