The Democratic U.S. Senate candidate who hauled in 89,000 votes as his party’s nominee two years ago is today languishing in obscurity in a three-way primary for the same office.

While Gov. Janet Mills and the former harbor master of a little seaside town, Graham Platner, hog the spotlight, David Costello of Brunswick is akin to a designated driver sipping a Diet Coke in the corner. 

Costello, who worked for years in government and offers a sensible liberal agenda, deserves more attention.

It’s no small feat that Platner, a 41-year-old oyster farmer and disabled combat veteran, has snatched the lead from Mills who, four years ago, racked up 376,934 votes to steamroll Republican Paul LePage to win a second term four years ago. But Platner’s thin resume and past penchant for saying nutty things on Reddit remain potential obstacles for him.

Nobody doubts that Mills, 78, has the experience for the Senate after serving as a district attorney, the state attorney general and governor. Some rightly wonder about her age. Others worry she’s too moderate.

As the competition between Mills and Platner grows ever more bitter, perhaps Costello, 65, can strike a different note.

In 2024, Maine Democrats tapped him as their standard bearer in a longshot race to unseat independent Sen. Angus King. To nobody’s surprise, Costello didn’t come close.

Although Costello remains mired in single digits in every poll and is trailing badly in fundraising, Jim Melcher, a professor of political science at the University of Maine at Farmington, told me there’s little question he is “a serious candidate” with sufficient policy and campaign experience.

Though Costello certainly has a challenging path to victory, Melcher noted he “hasn’t had the controversies Platner has had and he’s over a decade younger than Mills.”

Melcher expects Costello to influence the primary’s outcome in a ranked-choice voting race; his supporters’ second-place votes might help propel Mills or Platner over the top if there’s no first-round winner.

If voters might stop and recognize that Costello has experience and ideas worth considering, it’s possible he could do even better. As Melcher put it, if Democrats recognize the flaws in Platner and Mills, they may just decide the mild-mannered guy “hanging around trying to be the substantive alternative” may prove the most electable.

Costello recently told me that he felt Washington was broken, that Maine’s next senator needs to be able to articulate how that came to pass “and then lay out a phased approach” to fix it, or at least make it better.

“We need to have more people who are just talking rationally about politics,” he said, a comment that’s both clearly true and utterly depressing.

I don’t know who Democrats should vote for in the June 9 primary. But I am sure they shouldn’t simply ignore Costello. After all, he’s the only one of the trio vying for the nomination who has ever run for Senate before.