Tax reform legislation has finally won approval in both the House and Senate. Republicans now face the difficult task of merging the two different plans into a compromise bill.

And it won’t be easy.

The two measures are different in key areas and there are lawmakers in both chambers who are unwilling to accept some of the tax provisions introduced by their Republican counterparts across the Capitol.

House Speaker Paul Ryan, R-Wis., well aware of the differences that will have to ironed out, declined to speculate on what Senate provisions House lawmakers might accept or seek to change.

Lawmakers will return this week and immediately vote to establish a conference committee made up of Republican House members who will join senators tasked with hammering out a deal.

Republicans have rejected the idea of simply passing the Senate bill and sending the legislation to the president, locking tax reform into law by Christmas.

Senate Republicans passed their bill on Friday. The House passed its own tax reform plan on Nov. 16.

Both bills are similar in principle, lowering individual, corporate, and small business rates while eliminating many deductions.

“We are going to go to conference,” Ryan said. “This is such an important piece of legislation. It's generational-defining, and you don't skip the legislative process. You go through the regular legislative process, and that means we will be going to a conference committee.”

Differences are wide-ranging.

The Senate bill delays dropping the corporate tax rate to 20 percent until 2019, while the House bill kicks in the new rate immediately.

The Senate legislation holds onto the estate tax while the House bill phases the tax out over six years.

One of the most significant differences between the two measures involves Obamacare.

Senate Republicans tacked a provision onto their tax bill that would take away the penalty for those who don't purchase health insurance through Obamacare. In 2017, individuals who do not purchase health insurance owe the government $695, or 2.5 percent of household income, whichever is higher.

House Republicans have long been eager to repeal Obamacare, and the mandate is a central element that, if eliminated, would certainly unravel the law they have long despised.

The House GOP passed a bill to repeal and replace the law this year only to watch the Senate GOP kill the effort by failing to pass a bill of their own.

But House Republicans are wary of simply eliminating the mandate in the tax bill without a replacement to the law. According to analysts, dropping the mandate will raise premiums 10 percent thanks to fewer people signing up for plans.

“There are a lot of our members who would like to do it but there are some that don’t want to conflate healthcare and taxes,” Rep. Tom Cole, R-Okla., told the Washington Examiner.

Republican senators believe they have found a fix for those worried about premium increases by securing a promise from Majority Leader Mitch McConnell that the chamber will vote by year’s end on two measures to bolster Obamacare subsidies for insurers, which would lower premiums.

The Senate bill will also include a trigger that would raise taxes if the deficit grows too much due to tax cuts that are not accompanied by economic growth.

Many House Republicans have panned that provision and predicted it would not end up in a final compromise bill.

“I don’t think it survives conference,” Rep. Trent Franks, R-Ariz., said.

Some provisions in the Senate bill are attractive to House members.

The Senate bill caps the mortgage interest deduction at $1 million, double the House plan. “And I think we need to hold [onto] that,” Rep. Tom MacArthur, R-N.J., said of the Senate bill.

MacArthur also favors the Senate plan of keeping the deduction for medical expenses, which the House bill eliminates.

“I like what the Senate has currently,” MacArthur said.

Some Republicans, such as MacArthur, are eager to go to conference to secure tax provisions that did not make it into the House bill, including the deduction for state and local income taxes.

The Senate bill does not include that deduction and it is likely to be part of the negotiations beginning next week.

“There are some and there are changes the House felt would be made in conference, when it left here,” said Energy and Commerce Committee Chairman Greg Walden, R-Ore.