WASHINGTON — Now that the House and the Senate have both released their tax bills, the hard part can begin: getting them in sync.

For months, Republican leaders of the “Big Six” working group held weekly meetings so that the tax plans that they unveiled would be largely unified, making it possible for legislation to sail through Congress before the end of the year. But the House and Senate proposals diverge on important provisions that will be challenging for lawmakers to rectify in the coming weeks, in part because of competing political priorities facing lawmakers in each chamber.

Income tax brackets

Republicans originally wanted to collapse the tax brackets to three, from seven, in part to achieve their goal of simplifying the tax code so that people could file on a postcard. That proved difficult. The House plan went with four brackets: 12 percent, 25 percent, 35 percent and a top rate that stays at 39.6 percent for millionaires.

The Senate bill sticks with seven brackets of 10 percent, 12 percent, 22.5 percent, 25 percent, 32.5 percent and 35 percent but lowers the top rate to 38.5 percent for high-income individuals and couples.