Many political observers turned the page on the Democratic primary last month after Gov. Janet Mills suspended her U.S. Senate campaign. A series of polls showed her down by double digits to political newcomer Graham Platner, whose brass-knuckle populism is packing town halls across the state.

But David Costello, a 65-year-old Brunswick Democrat who has experience in federal, state and local government, continues to make the case to voters ahead of the June 9 primary that he’s the best one to take on Republican Sen. Susan Collins in the fall.

Costello has his work cut out for him. Some independent polls don’t even include his name, and those that do show him around 1%. He also trails in the money race, raising less than $145,000 through the end of March, compared to nearly $12 million raised by Platner and $13 million raised by Collins.

Many news outlets have all but written off his candidacy due to his lack of apparent support and his past unsuccessful runs — a dynamic his supporters blame for his low polling.

Costello also challenged U.S. Sen. Angus King in 2024, losing by 41 points. (Costello was the Democratic nominee in that race against King, an independent who caucuses with the Democrats.)

And Costello sought the Democratic nomination for the 2nd Congressional District in 2002, but lost to Mike Michaud.

During a virtual town hall event Wednesday, Costello was asked to distinguish himself from Platner, the presumptive nominee.

“I certainly think that I would be a harder person to attack than Graham is given … some of the comments he’s made,” said Costello, clearly hesitant to criticize the front-runner for his controversial past statements. “And the fact that I think that my experience compared to Senator Collins stands up very well.”

The primary winner will be determined by ranked-choice voting — a method supporters say allows voters to choose their favorite candidate without worrying about throwing away a ballot on a long shot.

But an instant runoff will only come into play if Platner doesn’t get a majority in the first round, which polling suggests he will easily do.

Although Mills suspended her campaign, she had not formally withdrawn from the race as of Thursday. That means her name will still appear on the ballot, and her votes will be counted.

A fourth candidate — Andrea LaFlamme, an adjunct college professor and union organizer — is running in the primary as a write-in candidate.

Costello said Thursday that he may have had a better chance to lure reluctant Mills voters if Platner hadn’t withdrawn from three televised debates and two others planned by the Maine Democratic Party after the governor suspended her campaign.

Still, Costello says, he’s a better candidate than Platner because of his education, his family story and his experience working in federal and state government.

He talked about how his father, a U.S. Army veteran and labor organizer, died when he was young, leaving his 28-year-old mother to raise him and his three siblings in Old Town, which once had a thriving papermill. At one point, they lived in a mobile home.

By comparison, Platner had a more privileged upbringing. His father was an attorney in Ellsworth, and his grandfather was a renowned architect and designer. He briefly attended the tony Hotchkiss School in Connecticut, before getting kicked out because of absenteeism. He later graduated from John Bapst Memorial High School, a private school in Bangor. After serving abroad, Platner attended George Washington University, but he did not graduate.

Costello’s educational background includes a bachelor’s degree from George Washington University and a master’s from the London School of Economics and Political Science.

Costello’s experience includes time working on Massachusetts Sen. Ted Kennedy’s presidential campaign in 1980; time with the U.S. Agency for International Development; his service as an aide for Maryland Gov. Martin O’Malley and his time as deputy and acting secretary of that state’s Department of Environmental Protection.

Platner, who served eight years in the Marines and Army, has never held elected office, though he served as the harbormaster and on the planning board in Sullivan.

Costello and Platner share similar policy views, including support for Medicare for All, universal childcare, increasing taxes on the ultra-wealthy and restoring full sovereignty for tribes in Maine. And both advocate for reforming the federal government to make it more responsive to people’s needs.

But perhaps the biggest difference between Costello and Platner is the enthusiasm behind their campaigns.

Platner is planning a Memorial Day weekend series of rallies with Sen. Bernie Sanders, which are sure to draw thousands.

Costello is planning more social media ads, and a series of virtual town halls like the one Wednesday.

Two dozen people logged on to hear him talk.