Barack Obama, the US president, had paid a rare visit to the House of Representatives to lobby for unity among his Democratic allies and reinforced it with a public speech.
Nevertheless, 39 Democrats still joined 176 of the chamber's Republicans in opposition to the proposal. One Republican broke ranks, nominally fulfilling Obama's vow to secure bipartisan support.
The bill stipulates that all Americans have to obtain health insurance and creates a government-backed insurance plan, popularly known as a "public option".
It would bar insurers from excluding people for so called pre-existing conditions and for charging more based on medical history.
Most employers would have to offer coverage to their workers - though some small businesses would be exempt and the government would offer subsidies.
Republican opposition
Republicans have been united in their opposition to the bill and have said that, if it was passed, it would open the door for an expensive government takeover of healthcare.
The fight to remake healthcare in the world's richest country now shifts to the US senate, where its fate remains unclear.
Harry Reid, the Democratic senate majority leader, struggled to pull together the 60 votes needed to ensure passage. He has hinted that the House may not act until next year.
That would make the proposed reform the main issue at the 2010 mid-term elections - when one-third of the senate, the entire House of Representatives and many US governorships are up for grabs.
If, as expected, the two chambers pass rival versions of healthcare legislation, they will need to thrash out a compromise version and approve it in order to send it to Obama to sign into law.
The US is the only industrialised democracy that does not ensure that all of its citizens have health care coverage.
However, Washington spends vastly more on healthcare - both per person and as a share of national income as measured by Gross Domestic Product - than other industrialised democracies.