After a brief flirtation with the GOP’s surprise Virginia Senate redistricting plan and fierce backlash from unions, a black House Democrat has soured on supporting it.

Del. Onzlee Ware of Roanoke announced on the House floor Friday that he has decided to vote against the new Senate map, which would create a new majority­-black district in Southside but also disperse black voting power in at least eight other districts.

He had said in an interview Wednesday that he was considering voting for it. His support would have provided Republicans with a measure of bipartisan support and helped blunt criticism that they had pushed the proposed districts through the Senate in a way that was offensive to blacks.

The map would probably lead to the election of a sixth black senator but also diminish Democratic power in the now evenly divided Senate. Ware had said Wednesday that he was open to the proposal because he was tired of putting the interests of black people second to the interests of the Democratic Party.

But he announced Friday that he had decided to vote against the map, which Republicans sprang on the Senate on Inauguration Day, when Sen. Henry L. Marsh III of Richmond — a Democrat regarded as a civil rights icon — was in Washington to attend President Obama’s swearing-in. Ware said he had concluded that the map, tacked onto a bill calling for minor “technical adjustments” to House districts, was too much of a departure from the original legislation.

“It was just too drastically changed,” Ware said.

Democratic legislators and union leaders pounced after Ware and another black Democratic delegate, Rosalyn R. Dance of Petersburg, said in interviews with The Washington Post that they were open to the map.

At the request of the Senate Democratic caucus, union members pushed the two hard, on the phone and in person, according to union officials and Capitol staffers.

“The whole Senate Democratic caucus was rightfully concerned that [their] vote was going to give Republicans cover and fix the optics,” said one union official involved in the effort but not authorized to speak publicly about it.

Dance, who lives in the newly created Senate district, did not address the matter on the House floor and could not be reached to comment.

But another union member, Julia Newton of Service Employees International Union Virginia 512, said that Dance told her she has decided to abstain from voting on the measure. The plan could come up for a vote in the House as early as Wednesday.

“She said she wasn’t going to vote at all,” Newton said.