Washington — Republicans will try urgently today to reconcile the tax overhaul bills they passed in the House and Senate, entering a delicate period where they have to retain the support of their party’s conservative and moderate members.

Party leaders insist that there are no showstopping differences between their two bills, each of which features a decrease in the corporate tax rate from 35 percent to 20 percent. Still, the bills feature differences worth hundreds of billions of dollars.

Lawmakers are expecting an intense period of work starting today as lobbyists descend on the conference committee that will negotiate differences between the two pieces of legislation. Of particular concern will be changes made hours before the Senate passed its final legislation early Saturday morning, when the Senate changed its bill to preserve a provision of the current tax code that sets an alternative minimum tax floor for very wealthy individuals. That provision would be eliminated in the House bill, and scrapping the alternative minimum tax has long been a priority for GOP tax writers.

The negotiations are to begin as Congress faces a Friday deadline to pass separate spending legislation or face a government shutdown, and for that task Republicans may need votes from Democrats. Yet Democrats are not at all eager to bail out the GOP on any issue, even keeping the government open, after watching helplessly as Republicans sidelined them to ram through a tax bill that is heavily weighted in favor of corporate America and the wealthy.

Republicans insist they will pass a final version of the tax legislation for President Donald Trump to sign before Christmas, and Senate Majority Leader Mitch McConnell, R-Ky., projected confidence on Sunday. And even Democrats acknowledged that in getting the tax legislation through the Senate, Republicans had gotten the toughest part over, given their failures in that chamber thus far this year on their other headline issue, repealing the Affordable Care Act.

“We’ll be able to get to an agreement,” McConnell said Sunday on ABC’s This Week. “I’m very optimistic about it. And we think this will make a big difference in getting our economy moving again and providing jobs and opportunity for the American people.”

Discussions are expected to continue throughout the week and could conclude as early as next week.