BELLEVILLE, Ill. — Since World War II, only two men — both Democrats — have represented the congressional district that runs from St. Louis' eastern suburbs through coal country, orchards and the Shawnee National Forest to the state's southernmost tip. But Republicans see the sprawling region as ripe for change.

U.S. Rep. Jerry Costello's abrupt October announcement that he won't seek another term after more than two decades — the longest tenure of any Democrat in Illinois' congressional delegation — has touched off a lively race among five potential successors in the March 20 primaries.

The 12th Congressional District could be the one spot where Republicans wrest away a seat in a year when Illinois Democrats hope a redrawn political map allows them to undo GOP gains in 2010 as part of efforts to win back a majority in the House. Congressional analysts say it is a critical seat for Democrats to hold, but Republicans are banking on the area's conservative leanings, angst over environmental regulations relating to coal and signs of frustration with Washington and Illinois' Democratic leadership.

"I'm just to the point where I want to see something different happen," said Sue Wilson, 65, a lifelong Democrat from the Union County town of Anna, population 5,000. "Voting Republican may not make it any better. I don't know. But I just think we're in so deep right now."

While Democrats say they are confident of maintaining the seat, the GOP takes heart from 2010 election results in which other parts of downstate Illinois shifted its way. Among them is U.S. Rep. Bobby Schilling's defeat of incumbent Democrat Phil Hare in the 17th Congressional District — turf not held by a Republican since 1980. A Republican snared a state senate seat that had eluded the GOP since 1975, and Quinn won only three of the Costello district's 12 counties in his bid for a full first term.

Some political analysts see the race as a toss-up.

"There's real potential for this one to flip, and Republicans certainly have a shot," said David Yepsen, director of the Paul Simon Public Policy Institute at Southern Illinois University in Carbondale. "This is an important, targeted seat for both parties."

The longtime incumbent's retirement typically attracted a flock of potential candidates, but the field is down to five contenders in the primaries.

On the GOP side are Rodger Cook, a former Belleville mayor and St. Louis Cardinals football player; Theresa Kormos, an O'Fallon nurse who narrowly lost the GOP primary in 2010; and Jason Plummer, a businessman who was the Republican candidate for lieutenant governor in 2010.

The Democratic field features Brad Harriman of O'Fallon, who is the former St. Clair County regional schools superintendent, and Ken Wiezer, a retired carpenter from Granite City. Chris Miller, an Iraq war veteran and Purple Heart recipient from Carbondale, announced Friday through a spokesman that he plans to abort his campaign and endorse Harriman within days.

In 2010, even as Republicans surged to take back the House, Costello got more than 60 percent of the vote, handily winning re-election. The district's majority has sided with the Democratic presidential nominee in the past three general elections. And the district cemented itself as a Democratic stronghold during the 44 years that Costello's predecessor — Mel Price — served in Congress until his 1988 death.

Southern Illinois is replete with culturally conservative "Blue Dog" Democrats, the kind that Costello repeatedly won over while he compiled an often-conservative voting record on such things as reproductive rights and gun rights. In line with coal-country interests, he has voted against some energy and environmental legislation.

At least publicly, Democrats don't show concern. The Democratic Congressional Campaign Committee insists the race isn't among their top worries in Illinois. And Harriman, who has been endorsed by Costello, various unions and the Democratic parties of vote-rich St. Clair and Madison counties, figures GOP hopes are overblown.

He believes the district's blue-collar and union workers will stay true to Democrats.

"Diversity is our strength, but the district has remained working class throughout this time, and still is," said Harriman, 58. "I think it will remain in Democratic hands."

In Belleville, he most likely can count on voters like Mike White. As the lone employee of his own lawn-care business, the father of three once was a swing voter before loyally leaning Democrat since Bill Clinton's presidency. He says he's unlikely to change course now even with politics "one big mess."

"I'm going to keep my mind open, but most likely I'll vote Democrat," said White, 50. "I think the Republicans favor the rich, and I don't like that. I'm not rich."

But a mix of things could upset the status quo, including unemployment rates among the highest in the state, and voter alienation toward a dysfunctional Congress wallowing in approval ratings that are lower than the shafts in the region's coal mines. Yepsen says that "Blue Dog" Democrats like Costello "may be a dying species."

"It's difficult for a Democrat like that to get nominated these days because his party's so far to the left, much like a moderate might have difficulty as a Republican when that party's so far to the right," Yepsen said.

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Plummer, who works in his family's lumber business and narrowly missed becoming lieutenant governor, has the endorsement of U.S. Rep. John Shimkus, a Republican from the nearby 19th District.

"If you look at the 12 counties, the district is absolutely blessed with natural resources, a great workforce. There's a small-town fabric to it," he said. "But this district has been devastated simply because of poor public policy. There's no doubt we have a double whammy hitting us — the disaster of Springfield, and the problems coming at us from Washington."

Cook, the former Belleville mayor, said he's confident that the Blue Dog voters will go with a Republican in the general election come November.

"It's still a Democratic district if you look at the registration," he said. "In the general election, you're going to have to get the crossover votes, and those will be there for the right conservative candidate."