One vote.

That’s all it took to derail the Republican plan to elevate a third African-American to the seven-member Virginia Supreme Court.

That surprising vote to block Judge Rossie D. Alston Jr. from joining the state’s highest court came from a member of the GOP caucus — retiring Sen. John C. Watkins, R-Chesterfield.

He cast that crucial vote during Monday’s special session of the legislature that Gov. Terry McAuliffe had called to allow the legislature to rework illegally drawn congressional districts — a project that also collapsed in partisan discord. The collapse ensures that a federal court will handle the redistricting chore.

In the 40-member Senate chamber, Sen. Watkins broke from his 20 Republican colleagues when Judge Alston’s nomination came to the floor. Instead, he voted with the 19 Democrats, creating a 20-20 deadlock on Judge Alston, or one vote short of the Senate majority needed for a judicial election.

The Democratic bloc included the state’s five African-American senators, Sens. Kenneth C. Alexander of Norfolk, Rosalyn R. Dance of Petersburg, L. Louise Lucas of Portsmouth, Mamie E. Locke of Hampton and A. Donald McEachin of Henrico.

(Democratic Lt. Gov. Ralph Northam cast a 21st vote against Judge Alston, but his vote was considered symbolic. The state Constitution expressly reserves votes on judges to elected members of the House and Senate. While the lieutenant governor presides over the Senate, he is not a member.)

Before the vote, Republicans expected to teach Gov. Terry McAuliffe a lesson by using their majorities in the Senate and 100-member House of Delegates to elevate Judge Alston from the state Court of Appeals to Virginia’s highest court. He would have joined current African-American Justices S. Bernard Goodwyn and Cleo E. Powell.

Judge Alston, who has served 16 years on the bench, was poised to replace new Justice Jane Marum Roush, the 22-year veteran Fairfax Circuit Court judge that Gov. McAuliffe appointed to fill a Supreme Court vacancy in late July, when the legislature was not in session.

If Republicans had succeeded, Judge Roush, 58, would have been the first appointed justice to be removed since 1900, according to state records.

That concerned Sen. Watkins, who felt the governor had appointed a qualified justice and worried about turning such appointments into a political football because of unbridled partisanship.

Judge Alston was the only judge interviewed for the Supreme Court post when Senate and House committees that handle nominees met Monday before the special session convened. Justice Roush was blocked from appearing on a party line vote.

Under tart questioning from Democratic legislators, Judge Alston, 58, pushed back against media reports that he failed to file required statements of economic interest for two years, producing evidence that he had done so.

Judge Alston, who acknowledged during the questioning that he votes Republican, also defended the unusual short sentences he had handed down in several high-profile cases while serving as a Prince William County Circuit Court judge before going on the Court of Appeals in 2009.

He told the committees that he sentenced a state trooper, who dropped a DUI charge in exchange for sex, to one day in jail for seven years because the prosecutor had poor evidence and believed the plea deal on the sentence was the best that could be done.

In a second case in which a child died after being left in a car on a blistering hot day, he said sentencing the father to limited jail time and long probation allowed Social Services to monitor the family and kept the man’s nine other children from being thrown into foster care.

Still, his chances of going on the Supreme Court are becoming slimmer.

Justice Roush will remain on the court unless the House and Senate replace her during the next legislative session.

Her current temporary appointment expires Sept. 16, which is 30 days after the special session convened.

However, Gov. McAuliffe has said he will reappoint her to the Supreme Court after the full General Assembly adjourns, expected by Aug. 31. The state Constitution does not allow the one or both chambers to stay in session to block the governor from making such recess appointments.

The governor could keep reappointing her so long as both chambers cannot agree on another candidate.

The judicial fight mirrored the Republican-Democratic division over redistricting. Any meager hope that the House and Senate could agree on a redistricting plan fell apart when the Sen. Watkins again joined Democratic senators to vote to adjourn their chamber, leaving the House unable to do anything.

After the Senate adjourned, Gov. McAuliffe called the special session “a waste of $400,000,” and announced he would leave the redistricting to the three-judge panel that ruled the 3rd District unconstitutional.

The court ultimately could give Republicans the win. Legal experts expect the court likely would make small changes to fix the 3rd District and would avoid the wholesale changes the Democrats want.