WASHINGTON — The first enrollment period for the Affordable Care Act ends at midnight Monday, closing one chapter on President Obama’s landmark health-care law and paving the way for a new round of confrontations that could ultimately determine the law’s long-term prospects.

Supporters face an array of political, financial and legal challenges in the coming months. Democrats and insurance industry officials are already seeking ways to blunt what may be the next big controversy: an expected increase in monthly insurance premiums next year for the health plans sold through the federal and state marketplaces.

Republicans, meanwhile, continue to use the law to attack vulnerable Democratic incumbents in the midterm elections, which will decide whether the Republican Party wins control of the Senate.

Combatants on both sides debated the administration’s report last week that 6 million people had signed up for private plans. “The law’s working,” White House senior adviser David Plouffe said on ABC’s “This Week.” He added: “And this was a seminal achievement.”

But Republican Sen. John Barrasso, Wyo., expressed skepticism about the figure. “They are cooking the books on this,” he told “Fox News Sunday.”

In the months and years ahead, other questions will loom: How will Americans react when they get fined next year for not having insurance? Will more states expand Medicaid under the law? And will the federal courts make future changes to the law, including barring the use of government subsidies to help pay for coverage in the federal marketplace?

The unresolved issues mean it is far too soon to know how President Obama’s signature domestic achievement – and one of the most polarizing pieces of U.S. social policy – will turn out. So far, the action has been “a warm-up act for what lies ahead,” said Larry Levitt, senior vice president of the nonprofit Kaiser Family Foundation.

Here’s a road map of what comes next:

Monthly insurance premiums almost always go up, because costs typically rise. So the big questions are: How much will premiums increase for next year, how widespread will the increases be, and how will consumers be affected?

Many plans “low-balled” prices for 2014 to attract customers. “It’s like opening day at the hardware store, and you’re going to have a special,” said Joseph Antos, a health policy analyst at the American Enterprise Institute.

Although some plans may continue to keep rates low to build market share, others probably will raise prices – perhaps to bolster profits or to compensate for getting a higher-than-expected percentage of older, expensive enrollees, analysts say.

During the first five months of open enrollment, younger people signed up for health plans at a lower rate than older ones. But some insurance experts say they are getting more young people than they expected; in any case, final demographic data are not in. The law provides some financial help for plans that end up with too many unhealthy customers.

Some industry officials are predicting double-digit increases in premiums in some states, but information about the new rates won’t emerge until later in the spring, the summer or later. Increases in premiums are likely to vary by health plan and location.

Even if premiums rise substantially, consumers who get federal subsidies to help buy insurance may not feel the blow. The subsidies, available to people with incomes between 100 percent and 400 percent of the federal poverty line, limit premium bills to a proportion of income.

That doesn’t help people who aren’t eligible for subsidies. Insurers say adding a cheaper level of coverage, as some Democratic senators have proposed, would help to attract healthier people who might not otherwise be motivated to buy coverage.

The political challenges for Obama and the Democrats may be even more daunting than the policy dilemmas – and have a bigger effect on the health law’s long-term future. If the Republican Party does well in congressional races in the fall, winning control of the Senate – and then the White House in 2016 – the law could be dismantled.

Republicans and their allies already are using the health law as a dominant theme. The conservative group Americans for Prosperity is spending more than $30 million on ads alleging that the law will increase what consumers pay for insurance and deprive them of access to the doctors they trust.

Several Democratic senators who voted for the law and will face the electorate this fall have suggested ways to improve the law. But congressional Republicans have shown no interest in legislative fixes, and National Republican Senatorial Committee spokesman Brad Dayspring said Democrats have a major liability.