A number of sticking points emerged last week as Republican lawmakers began jockeying for their favorite parts of the House and Senate tax plans.

Top tax writers from each chamber will formally meet Wednesday at 2 p.m. to discuss their differences, but the real negotiations have already begun behind the scenes.

House Majority Leader Kevin McCarthy said last week a finished tax bill likely wouldn’t be ready this week, so final votes could come just before lawmakers leave for the holidays. Congress is scheduled to remain in town until at least Dec. 22, when a stopgap government spending measure expires.

Republicans have until then to work out significant policy differences in the two plans if they still intend to send a final version to President Donald Trump before Christmas. Some of those differences appear to be already settled. House Republicans, for example, sound happy to accept Senate provisions rolling back the 2010 health care law’s so-called individual mandate and opening up drilling in the Arctic National Wildlife Refuge.

Watch: David Hawkings’ Whiteboard — How Two Bills Become One Law

Others could prove trickier to resolve. Here are six potential obstacles to a final GOP tax deal:

Alternative minimum tax

One provision likely on the chopping block is the Senate proposal to maintain the alternative minimum tax in some form.

The House-passed tax bill would eliminate the AMT for individuals and corporations. After some last-minute changes, the Senate bill ended up keeping the full corporate AMT and a scaled-back individual AMT in order to pay for other late changes in their plan.

Republicans in each chamber appear eager to ax the AMT in a final plan. Ways and Means Chairman Kevin Brady said House GOP members “feel strongly” about permanently ending the AMT, while conservative and business groups have zeroed in on repealing it as a top priority for conferees.

The trick for Republicans is to find other ways to make up the revenue, as they aim to keep the 10-year cost of the tax plan under $1.5 trillion, a limit prescribed by the budget framework they adopted at the start of the tax process.

Adding the individual and corporate AMT back into the Senate plan restored a combined $173 billion in revenue over 10 years, according to the nonpartisan Joint Committee on Taxation.

Pass-throughs

The two chambers came up with very different structures for taxes on “pass-through” business income, paid through individual returns by owners of subchapter S corporations, partnerships, sole proprietorships, limited liability corporations and other firms not structured as C corporations.

Finding a middle ground won’t be easy.

The Senate tax bill would allow a 23 percent deduction for pass-through business income, expiring after 2025. The House plan would cap the tax rate on pass-through businesses at 25 percent and implement rules aimed at making sure owners did not take advantage of the lower rate for nonbusiness income.

Brady has said there are “strengths” to both proposals. “Right now, we’re looking at how we can really improve on both structures, especially making sure pass-throughs that make a lot of capital investment in their business can achieve the lowest possible pass-through rate that we can deliver,” the Texas Republican said.

On the other side, Wisconsin Sen. Ron Johnson — a key Republican who spearheaded the push for a more expansive pass-through deduction — had mixed reviews for the House pass-through proposal, parts of which he called “crazy.” Johnson and another Republican, Sen. Steve Daines of Montana, have both shown they’re willing to withhold support for a tax measure if it doesn’t get the pass-through structure right in their view.

Influential small-business groups, who have actively weighed in on the pass-through provisions from the beginning, are sure to add pressure to the debate.

Corporate tax rate

Many of the changes Republicans are considering will be costly, so tax writers are also looking at ways to bring in more revenue to offset further losses.

One controversial way is to raise the proposed corporate tax rate from 20 percent to as high as 22 percent. Trump recently opened the door to such an increase, though White House officials have offered mixed messages since then about the president’s views on the matter.

Twenty-two percent would still be a major drop from the current 35 percent corporate rate, but plenty of Republicans and major outside groups view the 20 percent number as untouchable. Brady said last week that he remains “focused on 20” percent, despite talk of raising the rate slightly above what was in the original House and Senate plans.

Another issue they’ll have to settle is when the rate cut should begin. The corporate tax rate wouldn’t drop to 20 percent until 2019 under the Senate plan, while the House bill called for an immediate cut.

Senate Republicans have argued the one-year delay is less consequential than it appears, given other goodies for large corporations such as more generous expensing rules. Others are anxious for an immediate cut.

Again, the decision comes with major revenue implications — each percentage point cut in the the corporate rate costs about $100 billion over a decade, according to JCT, and delaying the 15-point cut in the Senate version gave tax writers an extra $125 billion to work with.

Child tax credit

Two Senate Republicans, Marco Rubio of Florida and Mike Lee of Utah, have advocated a more expansive child tax credit. They offered an amendment to the Senate plan that would have made the credit fully refundable against payroll taxes, paid for by a small increase to the proposed corporate rate and other changes.

That proposal was shot down on the Senate floor, but the two conservative senators are now gunning for a win in the final conference agreement.

If the conference committee “weakens” the child tax credit or further cuts the corporate rate without making the child tax credit “refundable for working families,” Rubio wrote Friday on Twitter, there are “going to be problems.”

The Senate plan increases the child tax credit to $2,000, expiring after 2025. The House proposal would raise it to $1,600 and create a new $300-per-person “family” tax credit for those not eligible for the child tax credit, expiring after five years.

Further expanding the child tax credit, especially if paid for by a smaller reduction in the corporate rate, is an idea with bipartisan support. It’s unlikely that any changes could persuade Democrats to vote for the tax plan in the end, but making such a change could help Republicans rebuff the main line of attack from Democrats that the tax plan is a giveaway to corporations and rich individuals at the expense of non-wealthy families.

‘SALT’ deduction

Republican leaders and tax writers were forced to backtrack from early plans to totally wipe out the deduction for state and local taxes, or SALT, after GOP members from high-tax states like New York and New Jersey revolted.

A compromise was reached to allow up to $10,000 in deductions for property taxes, a provision that was worked into both the House and Senate plans.

More changes are still on the table, and they could come with a significant price tag. Repealing most of the SALT deduction would bring in more than $1.2 trillion in new revenue over a decade under the House plan. The Senate plan would bring in less revenue because the changes would expire after 2025.

California Republicans, in particular, have sought an option to allow some form of deduction for state and local income or sales taxes — a more effective tax break for their constituents. Brady has said that’s one of several ideas in play to appease Republicans from high-tax states.

Any changes on SALT — or lack of changes — will affect GOP support for a final tax plan in the House, given the significant number of Republicans in that chamber from New York, New Jersey, California, Illinois and other high-tax states.

Individual tax brackets

The first step for House and Senate Republicans might be getting on the same page about individual tax rates.

It’s a central difference in the two plans that has largely been overlooked amid more controversial issues. But lowering individual tax rates and reducing the number of income brackets was a key plank in the initial tax framework Republicans agreed on earlier this year.

The House plan stuck to that goal, slashing the number of rates to four: 12 percent, 25 percent, 35 percent and 39.6 percent.

The Senate tax bill keeps a more complex rate schedule, with seven brackets beginning at 10 percent and topping out at 38.5 percent.

As Republicans try to frame their plan as a tax cut for nearly every American, they’ll need to start by drawing up income brackets that would make their promise a reality.

Mary Ellen McIntire contributed to this report.