WASHINGTON — Congress is closing in on a historic overhaul of the US health-care system, decades in the making, that would extend coverage to millions of Americans, curb insurance company abuses, and potentially rein in health costs.

President Truman, a Democrat, tried to establish a system to cover most Americans. So did President Nixon, a Republican, and President Clinton, a Democrat. All failed under fire from outside groups and weakened by rifts within the Congress.

This time, Democrats, convinced that success on this bill is vital to the Obama presidency, are going it alone. A party-line vote on such a sweeping bill is a high-risk strategy for Democrats. It exposes rifts in their ranks, and the deals that make a Senate bill possible could unravel what worked in the House.

Even with an 81-seat majority, House Democrats passed their reform plan with just two votes to spare. It’s even tougher in the Senate, where Democrats plus the two independents that caucus with them have a 60 to 40 majority. On paper, that’s the 60 votes they need to overcome a filibuster, but with no GOP support, they can’t lose a single vote. On Nov. 21, the majority won a key procedural vote to bring the bill to the floor by the narrowest of margins, 60-39.

Debate on the Senate bill is expected to begin Nov. 30. If the Senate passes its bill, here are potential flash points between the House and Senate:

A Public option
In a move that looked unlikely only a month ago, Senate majority leader Harry Reid included a public option in his 2,074-page plan, despite threats of a filibuster. Senate Finance Committee Chairman Max Baucus (D) of Montana has said that 60 votes did not exist in the Senate for a public option.

To woo moderates, the Reid plan includes an opt-out for states. The House bill does not. Moreover, the Senate plan could be further amended on the floor, if needed, to bring wavering centrist Democrats on board.

Several moderate Democrats have already said that the public option in its current form is a nonstarter. Sen. Joseph Lieberman (I) of Connecticut, another vote the majority must count on, says that a public option is unacceptable to him - in any form. Vermont Sen. Bernard Sanders, the other independent who caucuses with Democrats, says that he won’t vote for a bill that weakens the public option. Likewise, Speaker Nancy Pelosi and liberals in the House Democratic caucus see a robust public option as an essential element of any healthcare reform. Without it, they say, there will not be enough competition to drive costs down. Members of the House progressive caucus have threatened to vote down any bill that does not include a strong public option.

Raising taxes/cutting costs
Both the House and Senate plans count on billions in savings by cutting waste in Medicare and taxing healthcare providers. But the search for additional revenues is opening Democratic rifts.

The House bill adds a surcharge for high-income taxpayers. The Senate bill does several things: It adds a Medicare payroll tax on the wealthiest Americans, it taxes high-end, so-called “Cadillac” insurance plans, and it empowers an independent board to propose cuts to Medicare if savings targets are not met. The cuts take effect unless Congress finds comparable cuts elsewhere.

Critics see the independent board provision as punting hard choices. “But it’s better than nothing, and that’s what the House bill has - nothing,” says Maya McGuineas, president of the Committee for a Responsible Federal Budget.

The bigger rift within the Democratic caucus is likely to be about the Senate tax on high-end insurance plans. Unions oppose this plan because it would have an impact on hard-won contracts. “We will work hard to eliminate this provision,” said Richard Trumka, president of the AFL-CIO.

Individual mandates
Individuals are required to purchase insurance policies for themselves and their families in both House and Senate versions of the bill, but the bills differ on enforcement. Penalties in the House version of the bill include a fine of up to 2.5 percent of a person’s adjusted gross income and as many as five years in jail. Exceptions would be given to those who cannot afford coverage, religious objectors, illegal immigrants and those not covered for less than three months.

But Senate Democrats stripped criminal penalties out of their bill. Instead, individuals who fail to purchase insurance face a $95 fine beginning in 2014, which increases to $750 by 2016. Senators were motivated by the prospect of GOP ads blasting Democrats for jailing parents who didn’t meet the mandate.

Enforcement is a key issue. Without strong enforcement, insurers say, many people will pay the fine and opt out, depriving insurers of money they need to cover people with preexisting conditions.

Abortion
The House bill includes an amendment by Rep. Bart Stupak (D) of Michigan that toughens restrictions on funding for abortion services. The compromise proposed by Senator Reid bans federal funding for abortions and requires the secretary of Health and Human Services to verify that taxpayer dollars aren’t directed to pay for an abortion. But, unlike the Stupak amendment, the Senate plan does not bar insurance plans that cover abortion services from the new health insurance exchanges. The National Right to Life Committee calls the compromise “contrived.”

But it may offer a middle ground. “The abortion language is not the language I would like or that Bart Stupak would write, but I do think this is a sensible position that we could accept,” said Sen. Barbara Mikulski (D) of Maryland.