Rep. Frank Wolf‘s decision to retire came as a surprise — as recently as last week, leaders in both parties fully expected the Republican to run for an 18th term. (Manuel Balce Ceneta/AP)

U.S. Rep. Frank R. Wolf announced Tuesday that he will not run for reelection in 2014, ending a distinctive three-decade career in Congress and instantly making his bellwether Northern Virginia seat a prime battleground in next year’s midterm elections.

Although the 74-year-old Republican has been a perennial subject of retirement rumors, his decision came as a surprise: As recently as last week, leaders in both parties fully expected him to run for an 18th term. But in a statement issued by his office, Wolf said he plans to turn instead to his longtime work on humanitarian issues.

“As a follower of Jesus, I am called to work for justice and reconciliation, and to be an advocate for those who cannot speak for themselves,” Wolf said. “I plan to focus my future work on human rights and religious freedom — both domestic and international — as well as matters of the culture and the American family.”

Wolf has been a vocal and sometimes lonely advocate for oppressed religious minorities, particularly Christians in Egypt, Syria and Pakistan. And although he is conservative on many issues, he has been willing to defend federal workers, squabble with anti-tax activists and cooperate with Democrats, making him something of a rarity among modern House Republicans.

Wolf’s decision came just a week after Fairfax County Board of Supervisors member John W. Foust (D) said he would run for the House seat.

Rep. Frank Wolf campaigns in Leesburg’s historic downtown in July 2012. Wolf’s 10th district stretches from McLean west along the Dulles Toll Road, through Winchester to the West Virginia border. (Melina Mara/The Washington Post)

Wolf has beaten a succession of little-known or underfunded Democratic opponents over the years, consistently performing better in the district than other Republicans at the top of the ticket. Wolf did not say that the prospect of a more competitive race influenced his decision, but Foust was widely expected to give him his toughest challenge in years.

Wolf’s 10th District stretches from McLean west along the Potomac River to the West Virginia border and takes in diverse swaths of affluent Virginia newcomers as well as large pockets of rural conservatives. It includes Loudoun County and portions of Fairfax and Prince William counties. Mitt Romney edged President Obama in the district by roughly 1 percentage point in 2012, and it is viewed as competitive by both parties.

Reps. Jim Matheson (D-Utah) and Tom Latham (R-Iowa) also said they would not run for reelection Tuesday, expanding the 2014 House playing field. Matheson’s district leans clearly toward Republicans, while Latham’s seat — like Wolf’s — is viewed as a battleground.

The race to succeed Wolf could turn on two competing narratives. Like their counterparts across the country, Virginia Republicans hope to capi­tal­ize on unhappiness with the rollout of the Affordable Care Act. But the 10th District was also hit especially hard by the October federal government shutdown, which many voters blamed on the GOP.

Aware that Wolf would retire eventually, Republican leaders in Richmond redrew the district after the 2010 Census to be slightly safer for the GOP. Upon hearing Tuesday’s news, some political analysts immediately pegged the 10th as a “tossup,” although Nathan Gonzales of the nonpartisan Rothenberg Political Report said he classified it as “Leans Republican.”

“A district that Mitt Romney won in a state that he lost overall shows me that this isn’t a pure tossup district,” Gonzales said.

Several Republicans are expected to consider running for the seat, which has not been vacant for a generation. Possible Republicans include Del. Barbara J. Comstock (Fairfax), Del. Timothy D. Hugo (Fairfax), state Sen. Richard H. Black (Loudoun) and businessman Keith Fimian, who lost two bids to unseat Rep. Gerald E. Connolly (D-Va.) in the neighboring 11th District.

Black said Tuesday that he was forming an exploratory committee and was “very seriously looking at” the race.

The Republican field could also include Artur Davis, a former Democratic congressman from Alabama who switched parties and moved to Virginia. Davis said Tuesday that he will “monitor the field that develops in the next several weeks in the hope that a responsible center-right candidate will emerge.”

Even Tareq Salahi, who ran for Virginia governor as a write-in candidate this year but is best known as the “White House gate-crasher,” said Tuesday that he would run for the seat.

One oft-discussed potential candidate, state Sen. Jill Holtzman Vogel (R-Fauquier), has already ruled it out. “I am absolutely not running for Congress,” she said Tuesday.

On the Democratic side, Foust’s opponents will, so far, be Fairfax lawyer Richard Bolger and Leesburg architect Sam Kubba. National party leaders were pleased when Foust entered the race, but other high-profile candidates may be drawn to the now-open seat.

Wolf has spent much of his 34 years in Congress climbing the ranks of the powerful Appropriations Committee, steering federal funds and projects to booming Northern Virginia. He has watched Loudoun and the Dulles corridor blossom from sparsely populated fields into a wealthy, technology-fueled driver of Virginia’s economy.

Wolf has long prided himself on his connection to his constituents. One of the few members of Congress who can drive home to his district each evening, he is a fellow commuter on Northern Virginia’s crowded roads and, unlike most of his Republican colleagues, an unapologetic booster of the federal spending that has fueled the region’s economic expansion and employed its workforce.

The 12 years since the Sept. 11, 2001, terrorist attacks have brought especially pronounced changes in Wolf’s district, as federal money has poured into public and private defense and homeland security ventures.

Wolf has also been an outspoken advocate for federal transportation spending, Most notably, perhaps, he partnered in 2008 with then-Gov. Timothy M. Kaine, a Democrat, to save the $6 billion-plus Metrorail extension to Dulles International Airport at a time when the Bush administration was threatening to kill it.

Wolf has traveled widely in his crusades for human rights causes, and that work was the subject of his 2011 book, “Prisoner of Conscience.”

In June 2004, Wolf went to the Darfur region of Sudan with then-Sen. Sam Brownback (R-Kan.). He traveled on a helicopter and in cars for more than eight hours so he could see for himself the burnt villages and displacement of thousands and meet victims of rape.

During the visit, government minders warned the women at a musty school, filled with refugees, to stop talking about the rapes or face beatings. Minders also were seen handing out bribes to keep women from speaking to foreign visitors.

But the women spoke anyway.

A group of people handed a journalist two letters, written in Arabic, that listed 40 names of rape victims and wanted the list to be sent to Wolf, who was pressing the government to disarm the government-backed militias known as the Janjaweed.

When he saw the list, Wolf was moved to tears. This sort of trip, he said, “was among the most important things I can do a public servant.”

John Prendergast, co-founder of the Enough Project, a human rights group, said that Wolf “was a champion with few equals for human rights issues. Though it gained him very little politically, Rep. Wolf worked tirelessly on behalf of people suffering in conflicts and dictatorships around the world.”

Wolf has been a fervent believer in independent inquiry, backing the creation of the Iraq Study group and pushing — unsuccessfully — for a similar group to study the U.S. presence in Afghanistan and a select committee to probe last year’s attack on the American diplomatic mission in Benghazi, Libya.

On economic issues, Wolf’s willingness to keep all options on the table to battle the growing federal debt has sometimes drawn the ire of the right.

Wolf has refused to sign the anti-tax pledge promoted by Grover Norquist’s group, Americans for Tax Reform. In 2011, Wolf gave a lengthy House floor speech “to voice concerns I have with the influence [Norquist] has on the political process in Washington” and to accuse Norquist of associating with “unsavory people.” (Norquist responded that Wolf was simply “frustrated and irritated, and it’s sad.”)

Sen. Mark R. Warner (D-Va.) praised Wolf for his willingness to work across the aisle, including when Warner served as governor.

“We have worked closely together on Northern Virginia transportation issues and partnered in consecutive sessions of Congress on bipartisan legislation that would encourage the on-shoring of jobs back to Virginia which have moved overseas in recent years,” Warner said. “Frank has also been a passionate advocate and reliable ally in my ongoing efforts to find common ground on issues surrounding our nation’s deficits and debt.”

Connolly said in a statement that it had been an “honor” to work with Wolf.

“Frank has been a leader on rail to Dulles, a tireless champion of federal workers, a partner in gang prevention, and a passionate advocate for human rights around the world. Congress and Northern Virginia will forever be grateful for his service,” Connolly said.

Emily Wax contributed to this report.