Democratic Senator Harry Reid worked on the draft of the legislation for weeks
US Senate Majority Leader Harry Reid has unveiled legislation designed to overhaul US healthcare provision.
His proposed health care bill would extend coverage to 94% of eligible citizens, the Democrat said.
US President Barack Obama, who has made healthcare reform a domestic priority, commended the $849bn (£508bn) measure.
It has been criticised by Republicans, who said they would block it from becoming a reality. Debate on the bill in the Senate is expected to be fiery.
The legislation, which was outlined in a 2,074-page document, is said by Democratic aides to reduce deficits by $127bn (£76bn) over a decade and by as much as $650bn (£389bn) in the 10 years after that.
"Tonight begins the last leg of this journey," Mr Reid said, as he announced details of the measure late on Wednesday.
Under its terms, most Americans would have to have health insurance, while private insurers would be banned from refusing to provide insurance because applicants had pre-existing medical conditions.
Insurance would be made more affordable with subsidies available to help those in lower income bands, the Democrats say.
People would also be able to take part in new insurance market places and be able to choose to buy government-sold insurance from 2014, a provision intended to help regulate the prices charged by private companies.
Large companies would be required by law to provide coverage to staff. The costs would be covered by government cuts on future Medicare spending.
Mr Reid unveiled the legislation following a private meeting at which he outlined the legislation to Democratic senators who had voiced concerns about the bill beforehand.
The debate has already sparked strong emotions on both sides and initial manoeuvring is expected on the Senate floor later in the week.
Republican Senate leader Mitch McConnell said the bill had "been behind closed doors for weeks" and that the forthcoming debate would not be "short", the Associated Press news agency reported.
"Higher premiums, tax increases and Medicare cuts to pay for more government. The American people know that is not reform," he said.
Mr Reid will need the support of all 58 Democrats and two independents in the Senate if he is to gain the 60 votes required to pass the legislation without delaying tactics from the Republicans.
Mr Obama has already hailed a victory for his top domestic cause following the earlier passage of a health bill in the House of Representatives.
Passed in a narrow 220-215 vote, the House bill aims to extend coverage to 36 million more Americans and provide affordable healthcare to 96%.
The Senate now has to pass its own bill and the two must then be reconciled and voted on again before the programme can become law.
Mr Reid's bill differs to the House bill in that he calls for an increase of a half percentage point in Medicare payroll tax for people with an income of over $200,000 (£119,779) per annum - rising to $250,000 (£149,724) for couples.
There is also a tax on high-value insurance policies that is not contained in the House version of the bill.
If approved, the legislation could lead to the biggest changes in American healthcare in decades.