The Senate voted Wednesday to officially begin conference negotiations with House members over a tax code overhaul, as Republicans race to send a finished bill to President Donald Trump’s desk before Christmas.

The 51-47 party-line vote was largely a formality. Behind-the-scenes talks have already begun since the Senate passed its version early Saturday morning, following House passage of its own bill before Thanksgiving.

Central differences in the two proposals need to be resolved on individual tax brackets and rates, keeping or repealing the alternative minimum tax, the structure for taxing pass-through business income, the expiration of key tax breaks and more.

Several of those decisions also come with difficult math problems, as Republicans try to keep the final tax measure under the $1.5 trillion limit set by a budget framework previously adopted by both chambers.

Other differences will be less tricky to sort out. For example, House Republicans are expected to happily accept provisions in the Senate version that would repeal the individual mandate in the 2010 health care law and open up oil drilling in part of the Arctic National Wildlife Refuge in Alaska.

Further changes are also on the table beyond what was in either the House or Senate plans, like a possible tweak that would maintain certain limited deductions for state and local income or sales taxes.

Both chambers’ tax bills would have allowed up to $10,000 in property taxes to be deducted, but lawmakers from high-tax states, particularly California, have sought additional tax benefits for their constituents.

Some GOP lawmakers have said they’re confident they’ll wrap up a final tax package before the holidays. Congress is expected to remain in town through Dec. 22, when a continuing resolution released last week by the House Appropriations Committee is set to expire.

The House is set to take up that stopgap spending measure on Thursday. The Senate will need to pass it before midnight on Friday, when current government funding expires.