In Plymouth County, a high-stakes Senate race

State Representative Michael Brady (right) waved to cars passing by as he campaigned for an open state senate seat. Jessica Rinaldi/Globe Staff

A vacant state Senate seat in a district evenly divided between the parties is poised to become ground zero in the Republican Party’s aggressive attempts to make legislative gains — and the Democrats’ determination to dash the GOP’s dreams of a resurgence.

The high-stakes battle this fall for the Plymouth County-based seat, which was held for decades by Democrats until Senator Thomas Kennedy’s death in June, will be a magnet for cash-flush political action committees, labor unions, conservative and liberal activists from around the state, and even national partisans ready to spend money in local races.

It is a campaign that could give a huge boost — or a serious setback — to state Republicans, who, revitalized by several legislative successes and Charlie Baker’s election as governor last year, see an opening to gain more ground in their attempts to become a more potent force in Massachusetts politics. On Beacon Hill, the GOP holds just six seats in the 40-member Senate and 35 seats in the 160-member House.

The Second Plymouth and Bristol Senatorial District, which has grown increasingly Republican and conservative, is one of the most politically, economically, and culturally polarized Senate districts in the state. It is a case study of what the 21st-century Massachusetts political landscape has become.

It starts in Brockton, a heavily Democratic city with a majority of minority voters, mostly African-American and Hispanic. With 94,000 residents, the city has the majority of the district’s registered voters.

The district then moves east to the strongly Republican-leaning towns of Whitman, Hanson, and parts of East Bridgewater. It then takes in several other GOP election strongholds, Hanover to the north and Halifax and Plympton to the south. A chunk of Easton, just west of Brockton and evenly split between the parties, is also included.

The Plymouth district, in fact, is the sort of fertile ground for Republican growth that has developed in similar longtime Democrat-dominated regions around the Interstate 495 belt and Worcester County. They are areas that some political analysts call exurbia communities, populated with former conservative-leaning Democrats who recently migrated to the communities because of lower real estate prices.

Geoffrey Diehl jumped over items that are part of a yard sale as he campaigned for State Senator in Brockton.Jessica Rinaldi/Globe Staff/Globe Staff

But, increasingly alienated from their more liberal state and national parties, they are attracted to the antigovernment rhetoric of Republicans, which feeds into their grievances over taxes, their personal financial worries — and even the long commutes they have to make.

The state’s Democratic leadership, keenly aware of the demographic and ideological shift, is privately expressing alarm that the once-Democratic-dominated Senate seat is in serious jeopardy, according to party insiders.

The parties’ two leading candidates vividly illustrate the district’s divide. The Democratic leadership is rallying behind Brockton’s four-term state representative, Michael D. Brady, a local protege of the popular Kennedy who wears his local blue-collar upbringing and his pro-union passions on his sleeve. The 53-year-old lawmaker is proud of his glad-handing, constituent-attentive political style and is not shy about talking about his deep ties with the local and state Democratic political establishment. He faces a political unknown in the October primary.

Geoffrey G. Diehl, a 46-year-old three-term Republican state representative from Whitman, projects a sharply contrasting image. A Pennsylvania native and a sign company executive, he comes across as an earnest, clean-cut, antigovernment, antitax legislator who will appeal to the conservative suburban towns in the district. He’s a Tea Party favorite who recently attended a fund-raiser for Donald Trump’s presidential campaign.

While Brady is touting his deep political background, Diehl has been a thorn in the side of the establishment — mostly to his own party. His association with a band of dissidents looking to challenge Minority Leader Brad Jones so irritated the GOP leaders on Beacon Hill that they threw him off the House Ways and Means Committee earlier this year.

That rough edge has cost him some potential political allies and operatives who could be valuable in what will be a tough campaign, according to senior Republicans. Baker encouraged him to run, but party insiders don’t see the governor getting heavily involved in the race.

Diehl, instead, will have to rely more on the anti-establishment right flanks of the GOP to provide much of the resources for his campaign operations. Still, that outsider profile will play well in the heavily Republican-leaning towns, where GOP candidates can run up strong margins that offset the Democratic vote out of Brockton.

“I don’t care what people in power think I should be doing; I am just going to do what I think it is important for the people I represent,’’ Diehl said when asked about some of the hard feelings among his partisan colleagues.

Although he gets his best ratings from the fiscally conservative tax-cutting groups — and was a leader in the successful ballot petition that killed indexed increases in the state gas tax last year — he also bristles at being tagged as “antitax.”

“It’s not about being antitax; it’s about accountability and how government spends that money,’’ he said.

While the number of registered Democrats far outpaces Republicans in the district, the largest voting block is made up of unenrolled voters, and they tend to vote Republican. The recent exceptions were when the late senator Kennedy and former senator Robert Creedon, both moderate Democrats with deep local roots, were on the ballot.

State Democratic Party leaders and their labor allies are rallying behind Brady’s candidacy and are expected to pour money and effort into the campaign. Unions, particularly, are coming to his aid after he stood alone in the House in opposition this year’s state budget, citing a provision that seriously weakened a longstanding labor law.

“I have always considered myself . . . as the working guy, the blue-collar guy,’’ explained Brady in a recent interview. His strategy is clearly to use his deep-rooted experiences in Brockton politics and government to contrast his candidacy to Diehl’s budget-cutting positions.

“I have been a hands-on elected official. . . . I have been out there doing the work,’’ he said, noting that he had been both a school committeeman and city councilor in Brockton.

Diehl will likely get a big boost from several well-funded GOP conservative groups. The Massachusetts Fiscal Alliance, which kicked up a storm last fall with mailings aimed at Democratic incumbents, is expected to use its tax-exempt status to play a role. It spent more than $300,000 in legislative races last year.

A state political action committee, Jobs First, which is in good part funded by EMC Corp. heir Christopher Egan, will likely provide Diehl with support, according to sources familiar with its work. Egan’s PAC spent more than $410,000 trying to influence about 20 House races last year, defeating two Democratic incumbents and helping to secure eight open seats.

A national corporate-funded GOP group, the Republican State Leadership Committee, which plans to spend $40 million on legislative races around the country, has shown interest in funding races in Massachusetts.