OXFORD — The field of Democratic challengers vying for the right to take on U.S. Sen. Susan Collins next year grew bigger Monday when a retired U.S. Air Force major general, Jonathan “Tracer” Treacy, filed as a candidate.

“I’m going forward,” said Treacy, 61.

Treacy, who lives in Oxford, said he still has “a lot of homework to do” before making a formal announcement. But he decided to file the necessary paperwork Monday with the Federal Election Commission to make sure he did not do anything improper as he puts the pieces together for his campaign.

Treacy said recently he once admired Collins, a Republican, but soured on her after her votes for Supreme Court Justice Brett Kavanaugh and a $1.5 trillion tax cut that swelled the deficit.

Treacy joins state House Speaker Sara Gideon of Freeport, lobbyist Betsy Sweet of Hallowell and Saco lawyer Bre Kidman in the Democratic field.

Though Collins is already a formal candidate for re-election in 2020, she has said she will not make a final decision about whether to run again until the fall.

There is also an independent in the race, Danielle VanHelsing of Sangerville, and a GOP hopeful, Derek Lavasseur of Fairfield, who is aiming to defeat Collins in a primary.