President Barack Obama on Thursday dared Republicans to campaign to repeal his new health reform law, throwing down the gauntlet for mid-term elections with the words: "Go for it."

Obama poured scorn on opponents who have fired up their grass roots conservative backers with a vow to overturn the historic legislation which brings America close to universal health coverage for the first time.

"They're actually going to run on a platform of repeal in November," said Obama, launching a campaign to sell the new law to US voters, at a raucous rally in the heartland state of Iowa, the cradle of his 2008 campaign.

"My attitude? Go for it," Obama said, in remarks which marked the unmistakable opening shots in November's mid-term congressional election campaign in which Democratic control of Congress will be on the line.

"If they want to have that fight, we can have it because I don't believe the American people are going to put the insurance industry back in the driver's seat," Obama said in a sports arena at the University of Iowa.

The new battle over health reform reflects sharply differing views in each party over the political impact of the new law.

Republicans believe they can convince voters that the 940 billion dollar bill is unconstitutional, and a deficit-busting big government power grab that carves away at their basic freedoms.

Obama's Democrats bet that Americans will soon come to love the bill's strict regulation of insurance firms which have piled up a list of perceived abuses.

Liberated by the passage of the bill after a year of rancor, Obama fiercely mocked Republicans who predicted all kinds of disasters once it was enacted into law.

"Leaders of the Republican Party, they called the passage of this bill Armageddon! The end of freedom as we know it," Obama said.

"So after I signed the bill, I looked around to see if there were any asteroids falling, some cracks opening up in the earth.

"Turned out as a nice day. Birds were chirping, folks were strolling down the mall, people still had their doctors."

The president's choice of Iowa to launch his campaign blitz was no accident, as it was at the same University of Iowa campus that he first unveiled a plan for universal health reform in May 2007.

Obama's win the following January in the heartland state's nominating caucuses ignited his campaign and led him to eventual victory over his Democratic foe Hillary Clinton.

"This is the place where change began," Obama told the crowd, recalling the heady weeks of grass roots campaigning as his long shot campaign gathered pace.

The health care struggle was a reminder of "what so many of us learned all those months ago on a cold January night here in Iowa: that change, while never easy, but it's always possible.

"Yes we can, Iowa, Yes we did," Obama said, reprising his campaign slogan to argue he had delivered the change he promised.

In a clear sign that campaign season was nigh, the country song "Only in America" -- a staple of election rallies -- pounded out after Obama wrapped up, instead of formal presidential military march "Hail to the Chief"

The health care legislation will extend medical coverage to an estimated 32 million Americans who currently lack it and bans insurance company abuses as well as mandating that all US citizens buy insurance or face fines.

Back in Washington, the fierce partisan schism which prevailed through the bitter year of politicking it took to pass the bill, deepened, as Republicans threw out new blocking maneuvers to slow the health-care end game.

The Republican Party forced a new House of Representatives vote on a package of fixes to the health bill signed into law by Obama on Tuesday.

But the Senate Thursday passed a set of technical changes to the overhaul, sending the package back to the House final vote.

Senators had promised the House to make a set of amendments to the final law, and the House was expected to pass the package before the end of the week.

In a sign of just how heated the debate has become, Democrats have called in the FBI and the policy after more than 10 Democratic supporters of the bill complained they had received death threats and abusive or harassing messages.

But Representative Eric Cantor, the number two Republican in the US House of Representatives, warned some Democrats were "dangerously fanning the flames" by making public menacing messages and accounts of acts of violence.

"To use such threats as political weapons is reprehensible," said Cantor.