The Republican grip on Chester County government has existed for so long that many veteran politicos cannot pinpoint the date of its origin.

No wonder: County historians say the GOP reign can be traced back to the Civil War.

But before voters decide whether to sustain that 146-year tradition, they have to narrow the field of three Republicans and four Democrats running in the primary for county commissioner on May 15.

Two candidates from each party will advance to the general election on Nov. 6, when voters will select three to serve on the commission.

Traditionally, Republicans have taken a two seat majority, with a Democrat automatically getting the third minority-party spot.

This time, county Democrats contend they have the momentum. In the last year, they've won two state legislative seats long held by Republicans. And, unlike in previous campaigns, three of their four commission candidates have held elective office, and two have run county-wide before.

Democrats have previously made inroads in other suburban counties, with Chester County "the lone holdout" until recently, said G. Terry Madonna, director of the Keystone Poll and a professor at Franklin & Marshall College. He cited two key Chester County races last year, when Andrew E. Dinniman won a state Senate race, defeating fellow Commissioner Carol Aichele, and Barbara McIlvaine-Smith won a state House seat.

"I see what is happening in Chester County as having a huge impact on the future of state politics," he said. Republicans have long counted on GOP votes in the county to help serve as a counterweight to the strong Democratic vote in Philadelphia. "What happens in Chester County doesn't stay in Chester County, he said.

"For Republicans to have any chance to be viable statewide, they have to do something about the hemorrhaging of votes," he said. In recent years, GOP candidates in statewide races have regularly lost the once strongly Republican Philadelphia suburbs.

Michele Vaughn, chairwoman of the Chester County Democrats, says voters are ready for change.

"The Republican Party hasn't needed to be accountable because they've dominated," for decades, she said.

Republicans still have the voter registration edge - though it's no longer 2 to 1 - and they have a well-known incumbent seeking reelection.

Joseph "Skip" Brion, the county Republican chairman, said his party deserves credit for making Chester County a great place to live. He cited the county's AAA bond rating, low taxes, land preservation, and infrastructure improvements, such as the recent prison addition and the Justice Center that is under construction.

"That just wasn't by chance," he said.

Aichele, 57, of Tredyffrin Township, is running on that record as she seeks a second four-year term. Her colleagues, Republican Donald A. Mancini and Democrat Patrick C. O'Donnell, are not seeking reelection.

But there are signs of discontent within the party. Aichele received the GOP's endorsement, but only by a slim margin on the third ballot.

"That should have been a message to the party right there," said Sandy Moser, who is Aichele's unendorsed challenger in the primary. "Obviously, if it took three votes, there's unhappiness among committee members."

But Aichele counters that she's "hearing lots of positives about how effective the leadership has been over the last four years."

Publicly, party members obey the 11th Commandment: Thou shalt not speak ill of another Republican. Privately, there are grumblings about the party chair's role in choosing candidates who have lost recent races locally and statewide. Brion did not return calls to discuss that issue.

The GOP also endorsed Terence Farrell, 59, of Lincoln University, who is in his second term as recorder of deeds. Farrell is a Realtor, small-business man and former journalist. If elected, he would be the first African American commissioner.

Farrell and Aichele's priorities would be managing open space, promoting economic development and public safety, and keeping taxes low.

Aichele said she has about $60,000 on hand for this campaign, money left from her state Senate race, and Farrell says he has raised about $15,000.

This is the second primary challenge for Moser, 64, of East Brandywine Township. The president of Pennsylvania Republicans for Environmental Protection, she ran an unsuccessful campaign for commissioner four years ago. She says the current commissioners' open-space initiatives would not have occurred without her challenge.

Moser wouldn't say how much money she has raised - campaign-finance reports are due Friday - but counts among her supporters former Eagles coach Dick Vermeil, who is prominently featured on her campaign brochures.

Madonna said the GOP could possibly benefit if the general election candidates include the unendorsed Moser.

"That would negate the Democratic argument about insiders and the need for change," he said.

No Democratic candidate received enough votes for an endorsement at the party's nominating convention, but all say they have a commitment to open space, fiscal responsibility, and government accessibility and transparency.

Lawyer Virginia McMichael, 50, served eight years as an East Whiteland Township supervisor when the township enacted a moratorium on development. That experience, she told the Inquirer Editorial Board last week, has prepared her to promote county-wide efforts for transit-centered "smart growth," affordable housing, and open space.

Ken Knickerbocker, 51, is a software company executive who served eight years on the Octorara school board and, since 2004, has been president of the Parkesburg Borough Council. He told the Editorial Board that western Chester County is a "completely different world" from the more affluent eastern part and its interests need better representation on the board of commissioners.

Bill Scott, 62, is a lawyer who served two four-year terms on West Chester Borough Council, part of that time as president. He ran in the Democratic primary for commissioner in 2003.

Kathi Cozzone, 44, is an accountant and financial analyst from Uwchlan Township. Two years ago, as a political newcomer, Cozzone almost won the race for county controller against an experienced, well-known Republican.

McMichael and Knickerbocker each say they have raised about $35,000. Scott says he has raised $24,000, with $9,000 out of his own pocket, and Cozzone, $7,000.

Vaughn said one example of the shifting political scene was the fact that the county Chamber of Commerce was inviting Democrats to a meet-the-candidates forum for the first time.

"It wasn't a competitive political environment before," she said. "Today it is."

Contact staff writer Kathleen Brady Shea at 610-701-7625 or kbrady@phillynews.com.

Inquirer staff writer Art Carey contributed to this article.