Sen. Mark R. Warner (D-Va.) has agreed to support D.C. statehood, leaving House Majority Leader Steny H. Hoyer (D-Md.) as the only Democratic federal lawmaker in the region who does not want to see the District become the 51st state.

Warner, who is up for reelection next year, was a longtime holdout whose turnaround Wednesday has given statehood activists a boost.

Warner said he changed his mind after realizing that an increasing number of D.C. residents commute for work to Virginia. He previously had been worried that as a state, Washington could impose a commuter tax on Virginians driving into the nation’s capital for jobs. Warner said the reverse-commute pattern will only grow when Amazon expands into Northern Virginia.

“It was just time,” he said in an interview. “I do think the Amazon decision helps because we’re trying to continue to promote a regional economy.”

Amazon founder Jeffrey P. Bezos owns The Washington Post.

D.C. Mayor Muriel E. Bowser (D), who met with Warner on Capitol Hill as part of a lobby day for statehood, said that she had been courting him for a long time and that his reversal “completes the Democratic Virginians that are supporting D.C. statehood.”

She added: “We are not asking for a handout. We take care of ourselves in Washington, D.C. . . . We are demanding our rights as American citizens.”

The statehood bill — introduced by Del. Eleanor Holmes Norton (D), the District’s nonvoting representative, last month — has 198 co-sponsors in the House.

Sen. Thomas R. Carper (D-Del.) introduced the identical bill in the Senate on Thursday with 30 co-sponsors, all Democrats.

“Unfortunately, ‘taxation without representation’ is not just something students read about in history books; it is the current reality for the over 700,000 people living in the District of Columbia,” Carper said in a statement.

Hoyer, the influential second-highest-ranking House Democrat, who has spent nearly four decades in office, Wednesday declined to comment through a spokeswoman about his refusal to support the bill.

In coordination with House Speaker Nancy Pelosi (D-Calif.), Hoyer decides which bills go to the floor. He previously said he would not stop lawmakers from voting on statehood but declined to say if he would be one of them.

In an interview after Democrats won the majority last year, Hoyer said carving out a federal district and determining the boundaries of a newly created state would be complicated.

“However, I think it’s inexcusable and a blot on our democracy that 700,000 of our citizens don’t have full voting rights,” he said at the time.

Even if the statehood bill passes the Democratic-controlled House, it is unlikely to go anywhere in the Senate while Republicans hold the majority.

Warner, vice chairman of the Senate Intelligence Committee, signed on to the bill at a time when it is gaining support from Democrats with national ambitions and progressives who have framed statehood as a civil rights issue.

The House next week is expected to endorse the concept of D.C. statehood as part of H.R. 1, a sweeping package of voting rights, campaign finance and ethics reform initiatives.

Pelosi last month issued a lengthy statement support of statehood legislation.

Further, all of the senators who are running for president are co-sponsors, including Cory Booker (D-N.J.), Kirsten Gillibrand (D-N.Y.), Kamala D. Harris (D-Calif.), Amy Klobuchar (D-Minn.), Bernie Sanders (I-Vt.) and Elizabeth Warren (D-Mass.).

“Statehood is really a very solid part of the progressive zeitgeist of the Democratic Party,” said Bo Shuff, who leads the statehood advocacy group D.C. Vote.

In a CNN town hall Monday, Beau Finley, a Cleveland Park Advisory Neighborhood Commissioner backed by the Democratic Socialists of America, asked Sanders about the issue.

Sanders, who noted Vermont has fewer residents than the District, said it would be hypocritical to oppose statehood. He added that Republicans resist statehood because D.C. voters probably would elect two U.S. senators who are Democrats.

“I hope that my Republican colleagues do the right thing,” he said. “People . . . are entitled to representation in Washington.”

Activists said statehood is good for the region because future D.C. senators probably would provide two more votes to protect the Potomac River and Chesapeake Bay and advocate for Metro funding.

“[Warner] is being a good neighbor now, and that’s a good thing,” said Josh Burch, a grass-roots D.C. activist who voted for Warner as a student at Virginia Tech.

Warner was governor from 2002 to 2006, when his brand as a self-proclaimed “radical centrist” allowed him to court voters that Democrats had ignored in rural southwest and Southside Virginia.

The state has steadily moved to the left and reliably votes for Democrats statewide — a shift critics said Warner failed to grasp during his 2014 reelection campaign, which he narrowly won against Republican Ed Gillespie.