By Olivier Knox

WASHINGTON: President Barack Obama was preparing Tuesday to sign his historic health care overhaul into law at the White House before waging an all-out push to win over wary US voters ahead of November elections.

Obama was to sign the key legislation in the White House's East Room at 11:15 am (1515 GMT), as his Republican foes in the US Senate plotted political war on the last piece of his blueprint for near-universal coverage. Senior White House adviser David Axelrod said Obama had never been happier than late Sunday, when the House of Representatives passed the core of his plan.

"I haven't seen the president so happy about anything, other than his family, as long as I've known him and worked with him," Axelrod told CNN on Monday.

"Even election night he was not as excited as last night." On Thursday, Obama will launch the first of a series of campaign-style events on the bill's behalf in Iowa, the state that propelled him on the road to the White House.

Despite unanimous Republican opposition to the bill, House Speaker Nancy Pelosi defended her tactics to pass the controversial measure. "You strive for bipartisanship when you can. When you find your common ground, that's great. If you don't find your common ground, you have to stand your ground," she told ABC television.

The House also passed a stand-alone package of changes that Senate Republicans now plan to fight in hopes of bruising vulnerable Democrats ahead of the autumn ballot.

"Democrat leaders may have gotten their votes. They may have gotten their win. But today is a new day," said Republican Senate Minority Leader Mitch McConnell. "I have a message for our Democrat friends: Enough is enough."

Obama's rival in the 2008 White House race, Republican Senator John McCain, went further, vowing in an interview with a radio station in his home state of Arizona that "there will be no (bipartisan) cooperation for the rest of the year."

Speaking on ABC, he warned Democrats would pay "a very heavy price" politically and vowed Republicans would repeal the measure.

Overturning the plan was a mathematical impossibility in this election cycle because Republicans cannot win the two-thirds majority in the House and Senate needed to override Obama's veto.

And the Senate rules for taking up the measure make it unlikely Republicans can block it, though they could force a change that would require another politically difficult House vote.

So Republicans planned to offer some carefully targeted amendments aimed at turning up the heat on swing-district Democrats, then gauge their own members' appetite for a free-for-all fight against the bill, two party sources said.

McConnell signaled his intent to zero in on items like planned cuts to the hugely popular government-run Medicare program for the elderly, and tax increases on the wealthy.

Republicans also began announcing plans to fight some of the reform's provisions, including a requirement that Americans purchase health care, in the courts, challenging the constitutionality of the legislation.

Democrats, riding high after ushering in the most sweeping US social policy changes in more than four decades, said they welcomed the fight and dared their rivals to take a public stand on the measure's most popular items.

Obama's 2008 presidential campaign manager, David Plouffe, scoffed at the threat and vowed Democrats would not passively take their hits like a "pinata."

"We're going to go out there and not just talk about what we're for, but what the Republicans are voting against," he told ABC.

Republicans will target Democratic House members from some 50 districts carried by Republican presidential nominees McCain in 2008 and George W. Bush in 2004, said Tony Fratto, a former spokesman for the Bush White House.

"Those guys had a tough uphill climb to begin with. Some of them walked the plank and made tough votes... which put them in a tough position. And the health care votes will make it even worse," said Fratto, now a strategic consultant.

Democrats have acknowledged the challenge, rallying money and volunteers to help vulnerable party members - led by Obama himself, who says he is all too aware of the potential political cost.

And they have defied Republicans attacking the bill to explain to voters why they oppose provisions like banning the insurance company practice of dropping people from coverage for preexisting conditions or when they get sick.

Fratto said Republicans would avoid that minefield, focusing instead on taxes and spending.