WASHINGTON - This much is clear: Republicans finally have the muscle to wipe out key parts of President Obama's health law, the Affordable Care Act, and they plan to move quickly when Congress returns in January.

After that, the rollback creates major dilemmas for both parties.

While the GOP is united on gutting the law often called Obamacare, they have yet to agree on what they would do differently.

Some want to move as fast and aggressively as possible anyway. Others, including lawmakers from the Philadelphia area, hope for a two- or three-year transition period while they sort out a Republican replacement that will address the 20 million people who get coverage under the law.

Democrats, meanwhile, will have to choose between firmly opposing Republicans - leaving the GOP to own any consequences of their changes - and cooperating on a replacement in an attempt to preserve as many of the law's benefits as they can.

Neither party has a unified approach.

"We're not going to pull the rug out from somebody without having an alternative for them to move to, so we have some work to do, everybody knows that," said Sen. Pat Toomey (R., Pa.).

Republicans hope early next year to strike down key provisions that finance the law. However, the changes wouldn't take effect right away, and perhaps not until after the 2018 congressional elections, giving them time to draw up and approve a new plan, and delaying any political fallout.

Democrats slammed Republicans for not having a solution in place after years of criticizing the existing law. They say the GOP approach - repeal and wait - is a recipe for uncertainty that could disrupt health coverage for millions.

"They want their talking point that they repealed it, but they don't even know what their replacement is," said Sen. Robert Menendez (D., N.J.).Rolling back the law, however, is much easier than approving any replacement.

The GOP can use procedural rules to cut out the law's financial legs with a simple majority in both chambers, rather than the 60 votes typically needed to overcome Senate procedural hurdles.

Republicans hope in early January to take steps that tee up a repeal of critical provisions shortly after President-elect Donald Trump takes office Jan. 20.

After that comes the more complicated part.

Most Republicans don't want to suddenly end coverage for the millions who got health care under the law, including 479,000 in Pennsylvania and 398,000 in New Jersey.

In addition, other popular benefits could be politically hard to erase.

Many getting coverage under the law receive subsidies that help cover premiums: They average $248 a month in Pennsylvania and $322 a month in New Jersey, according to the Obama administration.

A much larger group - 150 million people with insurance through their jobs - also benefit from the law, Democrats argued.

It allows children up to age 26 to stay on their parents' insurance plans and ensures that women can't be charged more for having been pregnant. The law guarantees coverage for people with preexisting conditions, and bans lifetime coverage limits.

Democrats warned that Americans would rue losing those protections, and scoffed at GOP promises to keep them.

"This idea that somehow you can tear it out by the roots and still give everyone the kind of security that some Republicans claim they are for, I think it's really misleading," said Sen. Bob Casey (D., Pa.).

Republicans say they plan to preserve many well-liked features while offering a plan based on the free market, though they have yet to spell out how.

"It's prudent for there to be a soft landing on this," said Rep. Patrick Meehan (R., Pa.).

He and others said changes have to be made because of rising costs, skyrocketing deductibles, and decreasing choices.

For 2017, the average premium on the Affordable Care Act's marketplace in Pennsylvania will grow by 33 percent. The hikes are smaller in New Jersey, but insurers have abandoned the program in both states.

New Jersey has just two insurers on the Affordable Care Act's marketplace. The five-county area in Southeast Pennsylvania has one.

"The current system is imploding," said U.S. Rep. Tom MacArthur (R., N.J.). "We can't ignore it. They created a mess and we now have to clean it up."

For people with serious preexisting conditions, who often could not obtain health coverage or had to pay sky-high prices, several Republicans suggested creating a high-risk pool that insurers would bid on for coverage, and providing subsidies for people who need them.

Toomey trusts the free market to account for other features. He said, for example, that if many parents want to keep their children on their plans, insurers will respond to that demand.

Republicans also hope a new law can control costs, in part by eliminating mandates in Obama's plan. For example, Toomey said, a young single man might not need, or want to pay for, coverage now required for maternity care.

Passing a replacement through the Senate, however, will take 60 votes - and therefore some Democratic cooperation.

"My hope is that Republicans will not go too far too fast, and I will be a voice to press for that, and that Democrats won't become obstructionists, because everyone has a stake in getting this right," MacArthur said.

Democrats question Republicans' sincerity.

If the GOP was interested in a better plan, Casey said, they would have helped improve the law's flaws or offered an alternative by now.

"They're not serious," he said. "They've never been serious about replacing it with something that will work."

Democrats, though, have little chance of blocking a repeal of key provisions.

So, do they negotiate, and vote for the best package they can secure? Or force Republicans to pass a new proposal themselves, leaving the GOP to answer for its trade-offs?

"As far as I'm concerned, I did my job and made the hard decisions to provide health care for New Jerseyans and for the country," Menendez said.

If Republicans want a repeal, the burden, he and others said, is on them to offer something better.

Casey said that he would have "a high bar" for supporting any GOP plan, and that it would have to include the key protections that Democrats already enacted.

Republicans hope the reality of a repeal, and the prospect of millions losing health insurance, forces Democrats to come to the negotiating table.

"If it's going to help the people of my district and the United States of America, I'm going to work for it," said U.S. Rep. Donald Norcross (D., N.J.).

But, he quickly added, "If it's just a way to jerk us around, say it's bipartisan, I won't be a part of that."

@JonathanTamari