Santee, California (CNN) California Republican Rep. Duncan Hunter's devil-may-care attitude has been a good fit for parts of his rural and suburban San Diego County district for about a decade , matching the Wild West ethos and anti-government bent of his constituents.

Some voters found it amusing when Hunter exhaled plumes of smoke from his vape pen during two congressional committee hearings to protest a proposed ban on e-cigarettes on planes. They shrugged off the eye-popping bar tabs that appeared on his past Federal Election Commission filings. When the congressman, who was one of the first House members to endorse then-presidential candidate Donald Trump, made the aggressive suggestion that the US should pre-emptively strike North Korea, it was excused by some voters as the blunt talk of a hard-charging former Marine.

But in a year when control of the US House of Representatives could come down to one seat, it's evident from two dozen interviews here that there is growing unease among Republican voters about Hunter and the Department of Justice investigation into his campaign spending , which has stretched for more than a year.

The California secretary of state has now certified the list of candidates for the June 5 primary, meaning that if Hunter were indicted in the middle of this election year, the only path for another well-known Republican -- such as retiring Rep. Darrell Issa -- to replace him would be a write-in candidacy, according to state officials. Even if Hunter were to resign, the only way to remove his name from the ballot would be a court order.

The five-term congressman maintains his innocence. His aides and his lawyers did not respond to repeated requests for comment about the allegations for this story.

He has reimbursed his campaign account more than $60,000 since the FEC first questioned spending on video games in 2016, according to FEC records.

"There was wrong campaign spending, but it was not done by me," Hunter recently told local TV station KGTV-10News. His comments have put the spotlight on his wife and former campaign manager, Margaret Hunter, who also held and made charges on the campaign credit card. He told the San Diego Union-Tribune in April 2016 that he and his wife were the only holders of the credit card. At that time, he said he would be the only one to use the credit card in the future.

The cloud of suspicion around Hunter, and the huge amount that he's been forced to spend on legal bills, has buoyed Democratic hopes in the 50th District, even though Trump trounced Hillary Clinton here by 15 points.

Hunter's two most formidable Democratic challengers -- former Navy SEAL Josh Butner and Ammar Campa-Najjar, who was a Department of Labor aide in the Obama administration -- believe Hunter's legal entanglements, the growth of the Latino population here and the influx of younger military families have changed the calculus.

At left, former Navy Seal Josh Butner and, at right, Ammar Campa-Najjar, a former department of labor aide in the Obama administration are both challenging Rep. Duncan Hunter, a California Republican.

"In Hunter, you have a career politician who's been in a seat that was more or less handed to him by his father," Butner told CNN recently over coffee, referring to the elder Duncan Hunter, who was in Congress for nearly three decades. "I have a sense of service to put my country ahead of any party, or any personal profit or special interest. I don't think that's the case with him."

Butner, who spent 23 years in the US Navy, including combat deployments in Somalia, Iraq and Afghanistan, argued that the mere fact that Hunter is under federal investigation should give voters pause.

"As a sitting congressman, if you've gotten yourself into that situation, that's not a good thing," Butner said. "The lowest form of leadership is example. That's the starting point. Lead by example -- if you can do nothing else -- and he's not even doing that."

Hunter's actions roil voters, inspire challengers

Hunter's troubles have led strong Republican voters like Robert Luitjens, an 85-year-old Navy veteran from Escondido, to give the Democratic candidates a second look and others to say they may stay home in November. Luitjens said he is particularly dismayed by Hunter's conduct, because he wants Republicans to maintain control of the House this year.

"It bothers me that he's gotten himself into this position," Luitjens said, adding that he's tried to withhold judgment until the probe is complete. While Hunter has "represented his district well" -- like his father before him, Luitjens said -- "I think he's done some stupid things."

"You don't use the wrong credit card to do personal things," the Navy veteran said, referring to Hunter's initial explanations that his son had used the wrong card to pay for video games. "I know he claims his wife misused it; and one of his kids misused the credit card because they looked alike. But you should have control over those kinds of things. That bothers me."

Other voters are less forgiving of Hunter's conduct.

"I think it's disgraceful if the allegations are true. If I did that in my job, they'd fire me," said Republican voter Gil Hernandez, 61, during an interview at a hardware store in Santee. "I supported his dad; I supported him, but if everything comes down to where he's spending money the way he shouldn't, and if he's had this moral lapse, yeah, I think he should be removed. Should another Republican fill his place? Yes."

"I feel for his wife and I feel for his children -- if those allegations are true," Hernandez added. "But he was in the military. The guy should have known better. ... If you're a military guy, you'd think you'd have more discipline, more self-control."

While Hunter's conduct has provided an opening with voters, Campa-Najjar says he has tried to focus on Hunter's hard-right, pro-Trump message, which he believes is out of step with the new crop of voters in the district. The 30-year-old Latino-Arab candidate says his own working-class background will have greater resonance with voters.

"I'm not concerned about his misuse of campaign funds," said Campa-Najjar, who worked as a janitor while he was in school and served as a White House intern helping to select the constituent letters that President Barack Obama would read. "I'm concerned about his use of taxpayer funds."

"I don't think the message needs to be: 'He's corrupt, and have you seen the other guy?' I really don't like that message," Campa-Najjar said. He points to demographic shifts and the push to register Democrats in the 50th District -- including on six campuses throughout the district -- as a reason for Democratic optimism.

"Kamala Harris won this district when she ran for Senate, so a biracial progressive has won this district before," he said. "Then there is the reality on the ground that every day there's young couples, mixed families like mine, single mothers like mine -- they're all leaving San Diego because of gentrification in parts of the county that used to be affordable. ... They are leaving to go to East County and North County. And all those demographics I just described -- they are all Democrats."

Campa-Najjar says he's not running "to be the brown candidate," but rather as an advocate for middle class values. He cited Hunter's support for the President's tax reform plan, which the congressman acknowledged would hurt some California voters in his district, as an example of how he believes Hunter is favoring party over his constituents.

Still, the secretary of state's report at the beginning of this year illustrates the steep climb Campa-Najjar and Butner will face: Forty-two percent of the district's voters were Republican; 27% Democratic.

Does Hunter's position put him at odds with his district?

The 50th District stretches across eastern San Diego County from a portion of Temecula in Riverside County south to the ranch country below Rancho San Diego into the mountainous terrain around Alpine, where Hunter lives.

The median household income here is $71,692. Some 93% of families make less than $200,000. Veterans compromise about 10% of the voting age population, according to census data.

Hunter, 42, who worked as a business analyst after graduating from San Diego State University, quit his job after the September 11, 2001, terror attacks to join the Marines. He was elected in 2008 after serving in combat in Iraq and Afghanistan. His wife, Margaret, managed his campaign when he was called back up and deployed to Afghanistan in 2007.

The five-term congressman has said he is the subject of "slanderous, salacious" attacks within the press. He has also played on suggestions that the Justice Department is slanted against Republicans, by alluding to special prosecutor Robert Mueller's investigation of the Trump administration.

"I have trust in the justice system despite what's going on with Trump and Mueller, and the anti-Republican, pro-Democrat forces within the Department of Justice, at the highest levels, and the FBI," he recently told KGTV-10News, when asked about the allegations that he had misused campaign funds. "I'm optimistic on the outcome. ... It's been a year. That's a long time."

Republican operatives privately express frustration about the length of the probe, which began in 2016 when the FEC and the San Diego Union-Tribune questioned 68 charges on video games.

There were also campaign charges for vacations in Italy and Disneyland, dental bills, clothing purchases and unspecified items from a surf shop. A Hunter staffer explained to the Press-Enterprise newspaper that one $600 charge for "in cabin rabbit transport fees" was an accidental charge related to air travel by the family's pet rabbit.

A March 2017 search warrant showed that the FBI had searched the offices of Hunter's campaign treasurer in Alexandria, Virginia, seizing computers, documents and hard drives. Documents showed authorities were seeking any communication about disbursements between Hunter, his wife, several aides and the two employees at the firm that handled the books. The document retrieved by CNN from PACER later disappeared from public view, signaling that proceedings had moved on to a grand jury investigation.

The House Ethics Committee's recent announcement that it would continue to defer to the Justice Department probe into whether Hunter had used campaign funds for personal expenses was viewed by California GOP officials as one more sign that charges could be coming soon.