GONZALES, La. (AP) — Republicans vying for an open congressional seat representing much of coastal Louisiana are brawling over conservative credentials, with little to distinguish their platforms except the accusations against each other.

As Saturday’s GOP primary election nears, the mudslinging has escalated between the top two contenders in the 3rd District race, New Iberia lawyer Jeff Landry and former state House Speaker Hunt Downer of Houma, with the most recent attacks traded about their military service.

The third candidate in the GOP primary, Kristian Magar, an oil field manager from New Iberia, hasn’t raised enough campaign cash to get into the attack ad fights. But he remains a threat in the race, appearing in televised debates, Republican forums and tea party events.

If he can pull in enough votes, Magar could force an Oct. 2 runoff that would bleed off more money from Republican candidates and likely involve more attacks that bruise whichever candidate finally advances to the Nov. 2 general election to face Democrat Ravi Sangisetty, a Houma lawyer and the only Democrat to sign up for the race.

The 3rd District seat representing the mainly rural district that covers all or part of 13 parishes in south Louisiana was left vacant when Democrat Charlie Melancon decided to run for the U.S. Senate. The district stretches from St. Martin and Iberia parishes in south central Louisiana to St. Bernard and Plaquemines parishes in the southeast.

The three Republican candidates offer largely similar rhetoric. They decry federal government spending, complain of liberal leadership in Washington, take a hard line stance on illegal immigration, object to nearly anything President Barack Obama does and ally themselves with tea party positions.

At a tea party-sponsored forum in Gonzales, Landry described an ‘‘overreaching federal government’’ and Magar talked of a need to ‘‘take that gavel out of (U.S. House Speaker) Nancy Pelosi’s hands.’’ In a recent televised debate, Downer spoke of the need for ‘‘good conservative principles’’ in government.

Downer, a lawyer, began the race with heavier name recognition. He held a seat in the Louisiana House for 28 years before an unsuccessful 2003 bid for governor, was the state’s first veterans affairs secretary and is a retired major general in the Louisiana National Guard.

But Downer has run into complaints from some Republican stalwarts because of his Democratic past. He was a Democrat until 2001 and worked as legislative lobbyist for former Democratic Gov. Kathleen Blanco only a few years ago, after he switched to the GOP.

‘‘I’m the only candidate who’s been a Republican his entire life,’’ Landry said. ‘‘Until the Republican Party changes its fundamental beliefs, I will not leave it.’’

Landry has never held public office. He narrowly lost a state Senate seat in 2007 and before that worked as an aide to a former state senator from New Iberia.

Landry’s ads describe Downer as ‘‘a liberal hiding in conservative clothing’’ and blast his votes to renew hundreds of millions of dollars in taxes on food and utilities. They also call Downer a politician who switched parties to stay in office, and Landry has recently criticized Downer’s military record and called him a disgrace to the uniform.

Downer calls himself a longtime public servant rather than a career politician, and he said he has always been a conservative who worked in the Legislature to pass balanced budgets and supported Republican candidates even when he was a Democrat.

‘‘I believe in good conservative principles,’’ he said, calling himself a ‘‘Reagan Cajun’’ and noting, ‘‘Ronald Reagan was a Democrat longer than I was.

Downer has accused Landry of distorting Downer’s record and Landry’s military service. While Landry has called himself a veteran of Operation Desert Storm, Downer criticized Landry for not also noting that he never served overseas during his 11 years in the Louisiana National Guard. Downer has also publicized a lawsuit against Landry tied to a small business Landry once owned.

With his grassroots campaign effort, Magar has largely avoided the attacks. He bristles mainly when people suggest his campaign can’t succeed because of paltry fundraising.

Downer had raised more than $411,000 through Aug. 8, the latest campaign finance reports available from the Federal Election Commission. Landry had raised more than $533,000 through the same period, though he lent himself $49,000. Magar took in only $38,000 by comparison.

But the candidates are trying to sell themselves to a small group of voters.

While the district often trends Republican, voters must be registered with the GOP to cast ballots in the Saturday primary. Only about 86,400, or 21 percent, of the district’s 403,000 voters are Republicans, according to the Secretary of State’s office.