“I’d like to do it [repeal Obamacare] tomorrow, but reality is another matter sometimes,” said Sen. Orrin G. Hatch (R-Utah), left, with Rep. Tom Price (R-Ga.) last week in Washington. (Evan Vucci/AP)

Republicans on Capitol Hill are already laying the groundwork for a rapid repeal of President Obama’s signature health-care law beginning on the first day of the new Congress, before President-elect Donald Trump is even sworn in.

But the urgent efforts to make good on a Republican campaign promise six years in the making obscure major GOP divisions over what exactly to replace Obamacare with and how to go about it, and how long a transition period to allow before the law’s insurance would go away.

Hard-liners are pushing to move as fast as possible, bolstered by a GOP base eager to see lawmakers follow through on years of promises. But key congressional leaders are keenly concerned about potentially throwing millions off their insurance plans and repeating what they have long decried as Democratic missteps eight years ago, sparking a fierce political backlash by moving too far, too fast.

While Trump could sign legislation gutting the Affordable Care Act before the spring bloom, a full replacement could take months, if not years.

“I’d like to do it tomorrow, but reality is another matter sometimes,” said Sen. Orrin G. Hatch (R-Utah), chairman of the Senate Finance Committee, which will help lead the “repeal and replace” efforts. “We have to live with the real world. And the real world right now is that the Democrats won’t help with anything.”

Donald Trump has campaigned to repeal and replace the Affordable Care Act, otherwise known as Obamacare, once he gets into office. Now that he's won the presidency with a majority Republican House and Senate, that feat might not prove to be too easy. Wonkblog's Max Ehrenfreund explains. (Daron Taylor/The Washington Post)

Hatch and other high-ranking Republican senators are pushing for an extended transition period that could keep large portions of Obamacare in effect until 2019 or beyond, allowing time to carefully craft a replacement and push the final debate past the midterm elections.

Many of them, such as Sen. Susan Collins (R-Maine), have been chastened by conversations with insurers and state regulators who are warning of chaos in the market for individual insurance if Congress moves rashly.

“I don’t want to leave the 84,000 people in Maine who are buying insurance on the exchange uninsured because, all of a sudden, two-thirds of them who have subsidies have lost that subsidy,” Collins said.

But those Republicans are clashing with GOP colleagues — many of them sent to Congress in the midterm, anti-Obama waves of 2010 and 2014 — who see little reason to dawdle.

“The history of this place is, the longer it takes, the more exponentially the probability grows that it’ll never get done,” said Rep. Mark Meadows (R-N.C.), elected recently as the new chairman of the hard-right House Freedom Caucus. “Republicans have been saying they have a replacement plan for over two years, so why do we need three years?”

Part of the problem is that Republicans have never been able to agree on a replacement plan, despite railing against Obamacare for nearly eight years now. Their foot-dragging is a function of internal divisions and the political peril of floating a detailed alternative that would be closely evaluated for costs and benefits. Trump has also been vague, promising a “terrific” replacement that will provide “great health care at a fraction of the cost.”

Senate Majority Leader Mitch McConnell (R-Ky.) has announced plans to put an Obamacare repeal on the Senate floor on Jan. 3. (Jonathan Ernst/Reuters)

The current battle centers on when exactly to schedule Obamacare’s sunset. But other fights loom — over what precisely a replacement plan should look like, whether Obamacare’s Medicaid expansion will continue, whether lawmakers should also now tackle the future of Medicare and how Congress should assist insurers during the transition.

The battle lines, however, are familiar, with “establishment” Republicans on one side and conservative insurgents, mainly in the House, on the other. Those dynamics pushed GOP leaders into increasingly dramatic confrontations with Obama and last year helped force House Speaker John A. Boehner (R-Ohio) to retire.

Come Jan. 20, Obama will no longer serve as a foil for Republicans, and while Senate Democrats could block parts of a health-care overhaul, the real fight will occur inside the GOP.

Senate Majority Leader Mitch McConnell (R-Ky.) has announced plans to put an Obamacare repeal on the Senate floor come Jan. 3. But that will only set the stage for a future repeal bill that will have to tackle major decisions such as a sunset date and interim measures to stabilize insurance markets.

So far, Trump and key GOP leaders on Capitol Hill have shied away from taking firm positions on repeal and replace, but they have done little to tamp down the expectations of conservatives expecting a swift and wholesale substitute.

“We’re going to repeal Obamacare lock, stock and barrel,” Vice President-elect Mike Pence told donors to the conservative Heritage Foundation recently. “The number one priority of this administration is to keep that promise to the American people.”

Hard-liners on Capitol Hill see Pence and Rep. Tom Price (R-Ga.), Trump’s pick for health and human services secretary, as allies in their push for quick and decisive action. Pence called on Congress to pass a repeal bill “with all deliberate speed” but pledged only to then “set into motion a process to replace it.”

House Speaker Paul D. Ryan (R-Wis.) said Thursday that Republicans would move “as well and as fast as we can but make sure that the transition does not pull the rug out from under people.”

Transition spokesman Sean Spicer said Friday that Obamacare replacement strategy has been part of discussions Trump and Pence are having with top congressional leaders and that the talks are ongoing.

“The idea is to really figure out the sequencing on both the repeal and the replace,” he said.

Conservative activists who pushed a take-no-prisoners approach toward the Obama administration say their patience is limited.

“When Republicans have the House, the Senate and the White House, you don’t wait,” said Adam Brandon, president and chief executive of FreedomWorks, a conservative advocacy group. “I’m a Cleveland Indians fan. I only get a shot at a World Series every couple decades. When you have a shot to do it, you do it. That’s it.”

Democrats and many health-care experts are warning that a swift repeal could lead insurers to stop selling policies to individuals on federally mandated exchanges. More than 12 million Americans are covered under those policies.

Health and Human Services Secretary Sylvia Mathews Burwell briefed Senate Democrats on Thursday on the expected unraveling of Obamacare’s insurance exchanges, according to people familiar with her remarks inside the closed-door meeting.

“Delayed replacement is a situation where it is basically repeal and chaos in terms of what will ensue, because of the uncertainty that will get presented to insurers, providers, consumers and states,” Burwell told reporters after the meeting.

Republicans want to end Obamacare’s system of penalties and subsidies. But many — including Trump — want to continue to ban insurers from denying coverage or sharply increasing rates for the sick.

Experts warn that “repeal and delay” under those conditions would prompt insurers to flee the individual market. Linda J. Blumberg, a senior fellow at the Urban Institute, said insurers could face as much as $3 billion in losses if healthy individuals leave the market once the subsidies and penalties are eliminated.

“That $3 billion is the tip of the iceberg,” Blumberg said. “With all of these changes and the uncertainty, it is hard to predict how bad the risk is going to get.”

Keeping insurers in the market during the transition could require Congress to step in with bailout payments, something that would be deeply unpopular among conservative lawmakers.

“The insurance industry should understand that there’s a new sheriff in town,” said Rep. Ken Buck (R-Colo.), a Freedom Caucus member. “They signed up for Obamacare, and if they want to make a profit, they’re going to have to figure out how to make a profit in a free-market insurance industry.”

The strategy of repealing Obamacare without first replacing it reflects not only the wishes of the GOP base, but also Senate arithmetic.

There will be a slim 52-to-48 Republican majority come January, and while Republicans can gut Obamacare with a simple majority using arcane budget rules, passing a complete replacement will require 60 votes.

Only by dismantling Obamacare first, Republicans say, will they have a chance to persuade enough Democrats to support a replacement plan — and Democrats have already signaled that they will not do so.

“We all know we have to repeal it to get them to even settle down and work with us at all,” Hatch said.

Incoming Senate Democratic leader Charles E. Schumer (N.Y.), however, has rejected that strategy. “We’re not going to do a replacement. If they repeal without a replacement, they will own it,” he told The Washington Post recently.

But the Republican base, prodded by conservative media outlets and well-organized activist groups, has been loath to accept Democratic obstruction as an excuse for inaction.

One prominent activist, Jenny Beth Martin, co-founder of the Tea Party Patriots, suggested that Trump could goose the repeal-and-replace efforts by reversing an administrative ruling that gave financial assistance to 11,000 congressional members and staff who were forced onto the Obamacare exchanges under a compromise included in the original law.

“I don’t think anyone thinks that you’re going to flip a switch . . . but the replacement for it needs to happen quickly,” she said. “We want it done as quickly as it possibly can be done.”

Brandon was even blunter: “This is going to be a hard thing for Republicans, but tough [cookies],” he said. “They’re going to have to push this through using parliamentary maneuvers, and guess what? It’s hard.

“The political risk in doing nothing is why you got Donald Trump in the first place,” he added. “The old excuses of having divided government, they’re gone.”

A few strident Obamacare critics are urging activists to soften a bit. “They have to look at the complexity of the problem here, and hopefully they’ll recognize that it’s not quite so simple,” said Sen. Ron Johnson (R-Wis.), who was first elected in the 2010 tea party wave.

But many others are not.

“Literally every Republican member has made this part of their platform in running for Congress,” said Rep. Mark Walker (N.C.), a first-termer who will chair the conservative Republican Study Committee next year. “We’ve got to act on it.”

John Wagner and Amy Goldstein contributed to this report.