WASHINGTON (Reuters) - The Republican-controlled Senate Finance Committee approved President Donald Trump’s nominee to head the Internal Revenue Service on Thursday, despite Democratic opposition driven by a new Trump tax policy that protects “dark money” political donors.

The panel voted 14-13 along party lines to back the confirmation of Beverly Hills tax lawyer Charles Rettig as IRS commissioner. Rettig now faces a confirmation vote by the full 100-seat Senate, where Republicans have only 51 seats and the tie-breaking vote of Vice President Mike Pence.

Rettig’s confirmation had been expected to enjoy bipartisan backing. But Democratic support crumbled this week after the U.S. Treasury did away with a decades-old rule requiring some tax-exempt organizations, including political nonprofit groups such as the National Rife Association, to identify their financial donors to the IRS in confidential disclosures.

Republicans welcomed the change as a victory for free speech.

But coming hours after the arrest of Russian national Maria Butina on charges of trying to infiltrate a nonprofit group believed to be the NRA, the move stirred concern among Democrats and other Trump critics that the new policy could help foreign powers, including Russia, meddle in U.S. elections.

“We have a cloud that hangs over this whole administration,” said Senator Mark Warner, the top Democrat on the Senate Intelligence Committee, who also sits on the finance panel.

During his June confirmation hearing, Rettig assured lawmakers he would remain independent of Trump as IRS commissioner.

But on Thursday committee Democrats led by Senator Ron Wyden said they could not back Rettig, given the Treasury rule change and Trump’s failure to publicly confront Russian President Vladimir Putin about Russia’s meddling in the 2016 U.S. election during Monday’s Helsinki summit.

Senator Orrin Hatch, the committee’s Republican chairman, said Rettig had nothing to do with the policy change and added that no committee member had opposed Rettig up to now.

“This nomination is no longer an isolated debate,” countered Wyden, the finance panel’s ranking Democrat.

Wyden and other Democrats urged Hatch to hold a hearing on the new Treasury policy and to obtain Trump’s tax returns to determine whether the president has financial ties to Putin.

Wyden also called for committee oversight on the NRA’s role in the 2016 elections, alleged influencing peddling by former Trump lawyer Michael Cohen and the operations of the Trump Foundation.

“What the president calls a witch hunt is turning up a lot of witches,” Wyden said.