Maine Gov. Janet Mills is set to launch a campaign today against Sen. Susan Collins (R-Maine) in what will be a blockbuster midterm race.

Mills’ bid gives Senate Democratic leadership yet another recruitment success. But it also cements a messy intra-party brawl. Mills’ entrance in the race isn’t scaring off any of the current Democratic primary contenders.

We’ll start by listing the obvious: On paper, Mills is by far the most formidable Democrat running to unseat Collins. Mills is a popular, sitting two-term governor and her pitch will certainly lean on electability.

The other three main candidates — Graham Platner, Jordan Wood and Dan Kleban — have never held any elected office. So far, all three are pledging to continue their campaigns, even with Mills jumping in.

The Age Argument. If the 77-year-old Mills wins, she would be sworn in as a senator at the age of 79. Her younger opponents are using this against her.

Wood, who has reported raising over $3 million, said it’s time for a new generation of Democratic leaders.

“Janet Mills was my district attorney the year that I was born in Androscoggin County,” Wood told us. “This is going to be a very real primary, even if Chuck Schumer does not want it to be.”

Leaders We Deserve, a group co-founded by David Hogg to elect young progressives, and other Democratic youth organizations all jointly endorsed Platner Tuesday.

Platner has the most resources to really challenge Mills. His campaign has hauled in over $4 million and his populist messaging won him Sen. Bernie Sanders’ (I-Vt.) endorsement. But Mills’ supporters can point to Sanders, 84, as a way to sidestep the age argument.

The big question now is whether Mills will pledge to only serve one term and whether such a promise would neutralize the age argument.

Yet Mills has proven she’s willing to fight and won praise from the left when she publicly challenged Trump over his attempt to ban transgender athletes from women’s sports.

Meanwhile, Republicans are enjoying the prospect of a contested Democratic primary.

“While Democrats fight among themselves, Senator Collins will keep doing what she’s always done — delivering for Maine families,” Alex Latcham, the leader of the Senate Leadership Fund, said in a statement.

We should note that Collins hasn’t officially launched her reelection campaign, but has said publicly she intends to seek reelection and is actively raising money.