More than half of the registered lobbyists in Washington have worked on tax issues in 2017, a report showed on Friday, as lawmakers in the U.S. capital scramble to conclude a year-long effort to pass a tax reform bill before Christmas.

About 6,200 lobbyists, or 57 percent of those who reported activity in 2017, have been listed on disclosure forms as working on issues involving "tax" this year, the advocacy group Public Citizen said in the report.

Twenty companies and trade groups each hired 50 lobbyists to work on tax issues, the report said, with U.S. Chamber of Commerce hiring 100 and the Business Roundtable, an association of chief executives at big U.S. companies, employing 51 lobbyists.

The Republican bill would be the biggest overhaul of the U.S. tax system since the 1980s and its ultimate shape and success are crucial to U.S. corporate profits over the next 10 years.

As drafted, the Senate bill would cut the U.S. corporate tax rate to 20 percent from 35 percent after a one-year delay and reduce taxes for some businesses and individuals, while ending many tax breaks.

Democrats have been united in their opposition to the bill, calling it a giveaway to the wealthy and corporations.

U.S. companies have a vested interest in nearly every aspect of the Republican tax plan, saying they would use a tax reform windfall to buy back shares, retire debt or invest back into their business.

Five large U.S. companies have hired at least 15 lobbying firms for tax issues in 2017, the report said. Comcast hired 23 firms, Anheuser-Busch brought on 19 companies, Verizon Communications hired 17, Microsoft hired 16 and Altria Group has engaged 15 lobbying firms.