The US House of Representatives is poised for a historic vote today that could bring into law the biggest changes to the country's healthcare system in half a century, and transform Barack Obama's presidency.

If approved, the $940 billion measure – the climax of a battle that has consumed US politics for nine months – would provide insurance to 32 million more Americans, extending healthcare coverage to 95% of the population. It would impose new taxes on wealthy and well-insured, and new rules on insurers.

Most uninsured people will now be obliged to buy insurance – the less well-off with the help of federal subsidies. In return for the extra customers, insurance firms will be barred from their most controversial practices, of refusing coverage to those with pre-existing conditions, and arbitrarily withdrawing cover from patients.

With Republicans unanimously opposed, the fate of the legislation hinges on some two dozen undecided Democrats. As of Friday night Obama was still short of the 216 votes needed for a majority.

On Friday, Obama, jacket off and sleeves rolled up, delivered a final impassioned appeal at George Mason University in Virginia. For a century, presidents since Teddy Roosevelt had been trying to provide coverage for all, he said. "Now we are at the point where we are going to do something historic this weekend." Obama continued: "If this bill fails, the insurance industry will continue to run amok. Are we going to let special interests win? Or are we going to make this a victory for the American people?"

So high are the stakes that Obama cancelled a long-planned trip to Indonesia and Australia that was to have started today. Instead he has been personally lobbying waverers, inviting them to the Oval Office for 15-minute pep-talks to try to win them over.

The Republicans are adamantly opposed. "We will do everything in our power to make sure that this bill never, ever, ever passes," said John Boehner, Republican minority leader in the House.

By one Friday estimate, the certain "No" votes totalled 206: all 178 Republicans, plus 28 Democrats. Everything depended on 20 or 30 House Democrats still officially undecided.

Their misgivings range from the objections of social conservatives to abortion language, which they say could permit government-backed cover for terminations, to complaints that the bill costs too much. Some Democrats are simply scared that a "Yes" vote will cost them their seats in November's mid-terms.

The abortion fears have been allayed by the support of Catholic nuns and other religious, and an even bigger fillip has been provided by the Congressional Budget Office, which reckons the measure will cost $940 billion over 10 years. But it will reduce the budget deficit by $138 billion over that period, and by $1.2 trillion in the decade after.

The wider public remains divided. A Wall Street Journal/NBC poll found only 46% of Americans in favour of reform, and 45% against.