Barack Obama tried to put his presidency back on course last night with a rare fighting speech to Congress rounding on his Republican critics and promising he would not be deflected from his ambitious plan to extend healthcare to all Americans.

With polls showing support for both the president and his healthcare plan slipping, Obama used a rare speech to a joint session of Congress to try to win over not only the public but sceptical members of his Democratic party who could decide the fate of his reform plan.

Apparently giving up on the bipartisanship he hoped for at the start of his presidency in January, he made little attempt to woo Republican members of Congress. Adopting an unusually aggressive tone for a president usually noted for his moderation, he expressed frustration with the way in which health reform was mauled by Republicans and other conservatives at townhall meetings across America over the summer.

His speech is likely to have alienated Republicans while liberal Democrats will be disappointed by an important passage in the speech in which he showed a willingness to drop the so-called public option, a federal-government insurance scheme.

Addressing Republican critics, he said he would work with those members of Congress prepared to offer alternative solutions. "But know this: I will not waste time with those who have made the calculation that it's better politics to kill this plan than improve it," he said.

Accusing Republican opponents of "scare tactics", he said: "I will not stand by while the special interests use the same old tactics to keep things exactly the way they are. If you misrepresent what's in the plan, we will call you out. And I will not accept the status quo as a solution. Not this time. Not now." Obama wants to extend health care to 46 million Americans who have no coverage.

On the public option, he said last night that a majority of Americans supported it. But signalling a willingness to drop it, Obama said: " To my progressive friends, I would remind you that for decades, the driving idea behind reform has been to end insurance company abuses and make coverage affordable for those without it. The public option is only a means to that end – and we should remain open to other ideas that accomplish our goal."

He expressed a determination to achieve reform, noting that presidents had been trying to achieve health care reform since Theodore Roosevelt a century ago. "I am not the first president to take up this cause, but I am determined to be the last," he said.

The Senate could vote on a bill before the end of the month and Obama has said he wants the legislation on his desk for signing by early next month. But that timetable is slipping.

He has much ground to regain after allowing his opponents to dominate the political agenda over the summer. An Associated Press poll that found that the number of those surveyed who are unhappy with the proposed reform has risen from 43% to 52% since July.

Obama, who earlier yesterday admitted he had made a strategic mistake by not setting out months ago what kind of bill he would like, allowing his opponents to exploit the ambiguities, last night began to sketch out the kind of health care bill he wants. He sought to allay the fears of those who already have health coverage that his reform will negatively affect their existing plans. "Let me repeat this: nothing in our plan requires you to change what you have," he said.

What he wanted, he said, was to ensure that the private insurance companies offered a better service, no longer able to deny coverage to people with pre-existing conditions, a big issue in the US. Insurance companies would also be required to cover routine check-ups, which many do not at present. His plan would offer "security and stability", the slogan for the night and the coming debate.

The White House invited a group of people from across the country who had been let down by insurance companies when they became ill to watch him deliver the speech. The main part of his bill would spread insurance coverage to the 46 million Americans who at present have none. Careful to say whether this would be a private or federal scheme, he said he would create a new insurance exchange that could negotiate decent deals with private insurance companies.

"Well, the time for bickering is over. The time for games has passed. Now is the season for action," he said.