Donald Trump promised voters an immediate repeal of Obamacare,but Republicans in Congress likely won’t have a bill ready for himon Day One. Or Day Two. Or perhaps even his first two weeks.

Related: GOP still struggling to develop ACAreplacement

Republican leaders will start deploying fast-track proceduresWednesday to get the bill through the Senate, but that will requireweeks of wrangling, if not longer.

It’ll be an early lesson for Trump in the sometimes-glacial paceof Congress. And it’s likely to get more difficult from here, asthe incoming president moves on to other areas where Republicansaren’t in such lockstep, such as infrastructure spending, where hemight need bipartisan support.

Vice President-elect Mike Pence told Republicans in a privatemeeting Wednesday that Trump, on his first day in office, plans totake action on Obamacare through executive orders aimed at makingsure the insurance marketplace isn’t disrupted by a repeal,according to Representative Chris Collins of New York, one ofTrump’s earliest House GOP backers. Republicans are hoping to havea replacement plan on paper in six months, Collins said.

Related: Why Senate Democrats could help replace theACA

On Wednesday morning, Trump warned Republicans about theirtactics on the health-care issue.

“Republicans must be careful in that the Dems own the failedObamaCare disaster, with its poor coverage and massive premiumincreases,” he wrote on Twitter. “Don’t let the Schumer clowns outof this web.”

“Massive increases of ObamaCare will take place this year andDems are to blame for the mess,” he added. ”It will fall of its ownweight -- be careful!”

While Trump talks of action with the speed of a tweet, thingshave a way of slowing down on Capitol Hill, even when party leadersand the president are on the same page.

On Wednesday, President Barack Obama and Pence visited CapitolHill for separate meetings on the health-care bill. Obama met withDemocrats to strategize over how to defend his signatureinitiative, while Pence talked with Republicans about the nextsteps in repealing and replacing it.

Obama told Democrats that he takes responsibility for not havingfully communicated the potential benefits of the health-care law,according to a Democratic aide.

“Despite the negativity you have a big chunk of the country thatwants this thing to succeed,” he said, according to the aide.“There are real lives at stake in this thing.”

Given Democratic opposition, Republicans plan to use a specialbudget procedure known as reconciliation, which is not subject to afilibuster, in hopes of putting a bill repealing much of thehealth-care law on Trump’s desk as soon as possible after his Jan.20 inauguration.

The process can get messy. It starts with passage of a budgetresolution with instructions to committees in each chamber, whichthen draft reconciliation packages. Those packages are combinedinto a bill that must pass both chambers. If the two versions havedifferences, they must be settled and passed again.

Republicans will first have to overcome a revolt from SenatorRand Paul on unrelated budget issues, with the Kentucky Republicansaying Tuesday in an interview he would oppose the budgetresolution because it adds significantly to the deficit.

"It never gets to balance. Not in 10 years, not in 100 years,not in 1,000," Paul said.

"Every Republican that was here voted for a balanced budgetamendment to the Constitution that said it should balance in fiveyears, but yet they are putting together a budget that neverbalances," he said. Paul added he isn’t swayed by the argument thatthe numbers don’t matter and are merely a way to fast-track theObamacare repeal bill.

Indeed, the resolution sees the deficit rising nearly every yearbefore hitting north of $1 trillion in 2026 alone, with thenational debt hitting $29 trillion that year.

Small margin

Paul noted that until Pence is sworn in as vice president andcan break ties in the Senate, a single other Republican voting nowould potentially force Republicans to rewrite or delay theresolution.

Related: GOP wants Trump to trim ACA benefits, says Congressaides

As part of the process, the Senate next week will hold avote-a-rama, where senators can call for votes on an unlimitedseries of amendments. That gives Democrats an opportunity to scoresome political points and force Republicans to take somepolitically awkward votes.

Senator Ben Cardin of Maryland predicted Democrats would keepthose amendments focused on Obamacare if Republicans narrowlytarget the budget resolution themselves, but said the strategywould be set by new Minority Leader Chuck Schumer of New York.(Democrats will get another bite at this process in the spring,when Congress considers next year’s budget.)

Once the House and Senate have adopted identical budgetresolutions, the committees have to draft the actual repeal bill ineach chamber. They’ll have to deliver $2 billion in deficitreduction over a decade set by the resolution, but otherwise arefree to write the details of the bill in their committee’sjurisdiction. The major limitation for lawmakers is that they can’tinclude items that only have an incidental impact on thebudget.

Thorny details

But the tougher piece will be agreeing on the details of howmuch to roll back and when. Republican need to keep at least 50 of52 of their flock in the Senate on board after Jan. 20, assumingPence will vote to break ties in their favor and no Democratsdefect.

A Republican senator on condition of anonymity said the detailsof the repeal bill remain very uncertain. Originally, Republicanswere planning to simply bring back the bill they put on Obama’sdesk last year for his veto.

But that bill was written knowing it wouldn’t become law, andnow some Republicans want to make tweaks to soften the blow ofrepeal.

"Even people who voted for this before are, ‘Wait a minute, waita minute, we knew that wasn’t going to happen,’" said the senator."There were no consequences." He said there’s a growing sense amongsome of his colleagues that they need to have a replacement forObamacare ready soon "because we’re going to own this."

"I think they’re stuck," Schumer said regarding the prospects ofRepublicans devising a replacement health-care bill. "They’re goingto regret the day they made this their opening issue."

The areas of uncertainty include how long Congress will have todevise a new system -- two years, three years or something else --and whether to keep some of the revenue and other savings fromObamacare in reserve to pay for a replacement.

"Anything when you get into details is tougher," acknowledgedSenator Mike Enzi of Wyoming, the chairman of the BudgetCommittee.

Split over timing

Republicans like Senator Lamar Alexander of Tennessee -- thechairman of the health committee, one of the two panels that willwrite the repeal bill -- have publicly urged party members to taketheir time and work with Democrats on a replacement. But there arealso voices that want a repeal and a replacement on a much moreurgent schedule.

"I think it should be very short. As short as possible," saidSenator Rob Portman of Ohio, of the transition to a new system. "Ifwe don’t I think you’ll see companies pull out of the exchanges,among other things."

Balancing demands for a timely repeal and replacement withsenators pressing for flexibility will be difficult, as any groupof three Republican senators on either side of the internal debatecould stall the process indefinitely.

Copyright 2018 Bloomberg. All rightsreserved. This material may not be published, broadcast, rewritten,or redistributed.