Del. Dave Albo (R-Fairfax) waves after a retirement speech on the floor of the House of Delegates in Richmond on April 5. (Steve Helber/AP)

Until this week, the Virginia House of Delegates district held by Del. David B. Albo for 23 years seemed like a lock in November for Republicans working to hold their solid majority in Richmond amid what Democrats hope will be a voter backlash against President Trump.

But Albo (R-Fairfax) surprised his party by announcing Wednesday that he won’t seek a 12th term — a move that has Republicans scrambling for a replacement in a vulnerable Fairfax County district where Democrats are ready to pounce.

“It’s going to be really difficult for Republicans to retain that district,” said Quentin Kidd, director of Christopher Newport University’s Wason Center for Public Policy.

“There is going to be a lot of headwind coming out of Trump’s White House,” Kidd said. “In the two and a half decades I’ve been watching Virginia politics, I haven’t seen Democrats as energized or organized for General Assembly races as I’m seeing them get now.”

Albo said he decided against seeking reelection for financial reasons. Taking time off from his work as a criminal defense lawyer in Northern Virginia for as many as 100 days per year to attend to business in Richmond eats into his ability to earn money for retirement and for his 11-year-old son’s college fund, Albo, 54, said in an interview.

“It’s been a struggle,” he said. “I’m not saying I’m going to be thrown out of my house or anything. But I’ve got to make money to properly care for my family.”

Since he took office in 1994, Albo’s 42nd District — stretching from West Springfield southeast to Mason Neck — has become increasingly Democratic.

Hillary Clinton won 57 percent of the vote there in last year’s presidential election. President Barack Obama, Gov. Terry McAuliffe and Sen. Mark R. Warner, all Democrats, also won their contests in the 42nd. Former Republican governor Robert F. McDonnell took 54 percent of the vote there in 2009.

Republicans say Albo has been able to hold his seat because he’s a strong voice in Richmond for Northern Virginia, helped by his stature as chairman of the House Courts of Justice Committee.

In 2013, Albo helped write the legislation that eventually became Virginia’s transportation funding law, which steers $6 billion toward highways, bridges and other projects that seek to lighten road congestion. About $1.9 billion of that money is meant for Northern Virginia.

Albo was also instrumental in finding money for the 2005 construction of South County High School in Lorton, and in 2002 he was behind one of the country’s first state anti-terrorism laws after six people from his district were killed in the 9/11 attacks. That law requires anyone arrested to be checked for immigration status.

“Dave has won by comfortable margins in a difficult district because he knows his community well,” said Fairfax County Supervisor Pat Herrity (R-Springfield), who has known Albo since they were students at West Springfield High School. “People like him because he tells it like it is.”

Whether Albo’s departure will affect the 66-34 Republican majority in the House of Delegates is uncertain.

But Democrats smell blood in the water.

House Democratic leader David J. Toscano (Charlottesville) characterized Albo’s decision to step down as part of “a rising tide of . . . resistance” in Virginia.

“Delegate Albo is now the eighth Republican incumbent in the House of Delegates to announce that he will either retire or seek other office,” Toscano said in a statement, which also called Albo “a wonderful friend” who has made “substantial contributions” during his legislative career.

“Meanwhile, we have 77 Democrats running in 49 districts currently held by Republicans, including all 17 that Hillary Clinton won,” the statement said. “This news speaks both to the strength of our candidates and the rising tide of the resistance.”

Albo conceded that his district is now vulnerable. He has yet to meet with anyone interested in stepping in, he said.

“It’s going to be a hard seat for Republicans to win, to be honest with you,” he said. “But the right Republican could win. If someone is a more community-minded Republican like I tried to be, I think that’s the right formula.”

Matt Ames, chairman of the Fairfax County Republican Committee, said there are several potential candidates in the district, though he declined to offer names.

“We’ll find a candidate that can win it with guidance and support from Dave,” Ames said. “It’s winnable.”

The two Democrats vying to compete for Albo’s seat in their party’s June 13 primary say Albo’s departure improves their chances in November.

Tilly Blanding, 66, a retired Fairfax County social worker, has been campaigning to oust Albo — among other things, highlighting his unwillingness to fight for Medicaid expansion in Virginia and calling him weak on women’s rights.

She vowed to stick to her message with the powerful incumbent out of the picture.

“I’m not going to work any less hard,” Blanding said.

Kathy Tran, a former U.S. Labor Department deputy administrator, praised Albo and said she’s the best candidate to fill his shoes — albeit with a progressive bent.

“I think he’s been someone who cares very deeply about this community,” said Tran, 38. “We’ve been given the opportunity for people in this district to vote for someone who has the same progressive values they share and who will stand up for them.”

Tran said she entered the race believing it could be a close election.

“But now it’s really competitive,” she said. “I’m really excited to be in the race.”