WASHINGTON - Republican U.S. Rep. Jim Gerlach announced Monday that he would not run for reelection, stunning political insiders and setting off a scramble to succeed the six-term congressman in November.

His decision turns what was considered a safe Republican seat based in Chester County into a top target for Democrats. With fewer competitive districts across the country, three of the House's toughest contests are now expected to play out in the moderate Philadelphia suburbs, setting up the region as one of the country's prime battlegrounds this fall.

Immediately after Gerlach announced his intentions, Larry Sabato's Crystal Ball, an election-forecasting website, moved his district to "toss-up" from "safe Republican."

The district is one of only 12 "toss-ups" nationwide, according to the site. Another is in South Jersey, where Republican Jon Runyan also has opted not to seek reelection. In Bucks County, GOP incumbent Mike Fitzpatrick is favored, but faces a battle in one of the country's most closely divided districts.

Democrats "need to compete in swingy suburban districts, especially because their old strongholds in Appalachia and the South ... are now moving out of their grasp," e-mailed Kyle Kondik, managing editor for Sabato's Crystal Ball.

The favorite in the race to replace Gerlach will depend on the candidates, Kondik said, but "for now, this is a very competitive race that both sides are going to have to invest in."

Said Pennsylvania GOP Chairman Rob Gleason, "I hate to see the seat open. We weren't planning on it."

In explaining his decision, Gerlach, 58, a member of the powerful Ways and Means Committee, expressed frustration with leaving his family behind during workweeks, only to see Washington spin its wheels.

"It's very difficult to see meaningful legislation move forward out of both the House and the Senate and get it signed into law, and I don't think that's going to change much in the next couple of years," he said in a conference call with reporters.

Gerlach crossed party lines on several critical votes in the last year. He said he had sought reelection two years ago, hoping fellow Republican Mitt Romney would win the presidency and change Washington's political dynamics, but now has seen little movement.

"We're just too split from a partisan standpoint," he said Monday. "I haven't seen any progress from President Obama's standpoint in terms of his willingness to lead and his willingness to work with congressional Republicans to get things done."

Gerlach, who has flirted with running for governor, left the door open to a return to politics.

"I'm not going to rule out running for public office in the future," he said, "but I have no plans right now to run for any public office this year."

Despite representing a relatively moderate district - it includes parts of Berks, Chester, Lebanon, and Montgomery Counties - Gerlach was considered a safe bet to win reelection, given his long history in Congress and the state legislature.

"I was surprised," said Gleason, the GOP chairman, who was out of the country and learned of Gerlach's news Monday morning. "We'll miss him."

Gleason called Gerlach the "anchor around which the Southeast Republican Party was built," but said a new candidate could energize the campaign and bring out suburban votes that Gov. Corbett will need this fall in his reelection fight.

Gleason dismissed the notion that Gerlach might take on Corbett in a primary, saying, "Nobody's going to run against Corbett."

Insiders in both parties wasted no time speculating on who was best positioned to jump into the Sixth District race. GOP names floated included State Sen. John Rafferty of Montgomery County, Chester County Commissioner Ryan Costello, and former State Rep. Sam Rohrer of Berks County. Val DiGiorgio, chairman of the Chester County Republicans and former county controller, said he was considering a run.

On the Democratic side, Michael D. Parrish, a businessman and former Army aviator from Malvern, has filed to run. In the past, he has donated money to Republicans, including $2,000 to Corbett's 2010 campaign and a total of $3,000 to GOP presidential candidates Romney and John McCain, records show.

Parrish was a registered Republican, but changed parties in December, said his campaign manager, Jocelyn Steinberg. Asked about the switch, Steinberg referred to a statement on Parrish's campaign website.

"There's no excuse for shutting down the government to make a political point," he said in a statement on the site. ". . . And there's no excuse for failing to create a better economic environment to help businesses grow and create jobs."

Parrish, a graduate of West Point, Stanford University, and the Wharton School of the University of Pennsylvania, heads Daleco Resources, an energy company with offices in West Chester.

While new electoral maps have made Gerlach's district friendlier to Republicans, the party cannot take it for granted, warned Alan Novak, a former Chester County and state GOP chairman.

"This has always been a very competitive swing seat," he said. "Whoever runs will have to run hard."