The sudden announcement that Rep. Bill Shuster, R-9th District, will not seek re-election this year opens the door to sweeping change in his district, with a competitive midterm election adding uncertainty in the months ahead.

Several Republicans could vie to replace Shuster, whose departure ends a 45-year Shuster family hold on the seat. Hours after Shuster’s announcement, repeat challenger Art Halvorson announced his plan to run.

With two months left for candidates to make up their minds and more than four months until the primaries, hopefuls in both parties could soon line up to join the fight.

Whoever wins could face a deeply changed Pennsylvania political landscape. National polls have shown the Democrats with a shot at a broad congressional victory in many states.

Shuster is one of many Republicans to step down or seek other offices this year.

“I think you could have considerable interest on both sides,” said G. Terry Madonna, director of the Center for Politics and Public Affairs at Franklin & Marshall College and a regular commentator on state politics.

The first Republican to officially seek the seat is Halvorson, a Manns Choice businessman and developer who has run against Shuster three times over two election cycles. Within hours of Shuster’s announcement, Halvorson said he would seek the seat in a written statement.

“I intend to complete my mission to bring conservative principles back into the heart of American governance,” Halvorson said. “America has a golden opportunity to reclaim its greatness, but President Trump urgently needs other outsiders like him to enter Congress and help him.”

Halvorson challenged Shuster twice in GOP primaries, coming within a percentage point of winning the Republican nomination in 2016. He won the Democratic nomination with write-in votes but lost to Shuster that November by a wide margin.

Halvorson has pointed to his so-called “Rescue America” platform, a series of legislative goals including term limits on Congress and a constitutional amendment to force a balanced federal budget.

“I will serve as a citizen-legislator so that I can fight fearlessly for the conservative principles America was built on,” Halvorson wrote.

In an interview Tuesday, State Sen. John H. Eichelberger Jr., R-Blair, said he is considering his options for a run and will make a decision in the coming weeks. He has not ruled out a congressional bid and said he has been in touch with conservative figures in Washington to discuss the possibility.

“It certainly does affect me,” he said of Shuster’s announcement. “I’m getting a lot of calls from people.”

Eichelberger said he was in a meeting at the state Capitol when Shuster made his announcement. He learned of the decision only when he returned to his office, he said.

“People were waiting in my office. They wanted to know what I was going to do. I said, ‘What are you talking about?'” Eichelberger said.

Eichelberger already made one attempt at Shuster’s seat — in 2001, when Shuster’s father, Bud Shuster, stepped down and triggered a special election.

“Now that the seat’s empty, I expect I’ll be hearing from more people asking me to run,” he said. “I’m going to be talking to my family and friends and praying about it.”

State Sen. Pat Stefano, R-Fayette, also has been named as a possible contender, according to news website PoliticsPA.

If Republicans keep their hold, there is a chance the winner could be more conservative than Shuster, a close ally to Trump. While Shuster has consistently voted with fellow Republicans, he has called for bipartisan compromises on some issues.

Halvorson has often attacked Shuster from the right, painting him as a “big spender” whose ideas were “backed by liberals.”

The American Conservative Union — a group that issues percent rankings of politicians — gives Eichelberger a 96 percent lifetime rating against Shuster’s 81 percent.

While the deeply conservative 9th District would be a tough battleground for Democrats, the party — newly invigorated by surprise wins and solid polling — could field serious candidates in a bid to replace Shuster as well.

“Let’s put it this way: We are entering a strange midterm election year,” Madonna said. “I think you can’t rule out some considerable interest.”

Fayette County psychologist Adam Sedlock — who ran in 2016 as a write-in candidate during a battle between Shuster and Halvorson — has also said he is running.

In November, Sedlock said: “Throughout the summer and into the fall, I met with the constituents of the Pa. 9th through picnics and banquets. I spoke with all citizens, regardless of their political party, who voiced their concerns regarding the direction this country has turned and of a Congress who has an approval rating lower than that of the president.”

Others could follow, and the nominee could receive more help from party leaders and activists than in past elections, Madonna said.

While both sides await a full list of primary candidates, Republicans statewide face their own concern: a wave of departures akin to Shuster’s and additional at-risk seats.

Democrats have shown an interest in targeting several eastern Pennsylvania districts, while Rep. Charlie Dent, R-15th District, has decided not to run again and Rep. Lou Barletta, R-11th District, is making a Senate run. Rep. Tim Murphy, R-18th District, stepped down amid a scandal involving an extramarital affair, paving the way for a March special election.

With a series of defeats and more seats set for hard fights, it is not surprising that some members of Congress have stepped down, Madonna said.

“Conservatives as well as moderate Republicans, there’s a growing attitude that this whole environment is not good. There is just something that I think is in the air,” he said. “They’re just unhappy with the culture in Washington, the bitterness, the polarization.”