One of the country’s most fiercely contested midterm races is fully underway.

Senator Susan Collins of Maine, a Republican seeking re-election, and Graham Platner, an oysterman and political newcomer who became the presumptive Democratic nominee last week, released dueling ads on Thursday in what amounted to a kickoff of the general election.

The ads are strikingly different, with Mr. Platner in full-throated attack mode and Ms. Collins pointing to what she has done for Maine — underscoring what are likely to be some of their central arguments. The spots represent the start of a flood of political spending in Maine, with the race already on track to be one of the most expensive in state history.

Here’s a look at both ads and what the candidates are trying to do with them:

Platner argues that ‘Susan Collins’s charade is over’

Mr. Platner’s fiery ad aims to capture the energy around his insurgent progressive campaign, while casting Ms. Collins as old, out of touch and in the pocket of President Trump and his allies.

“Susan Collins’s charade is over,” Mr. Platner declares in his booming voice as the ad begins, later adding: “A performative politics that enables the destruction of our way of life is disqualifying as a United States Senator. I am running because it is time for change.”

He attacks Ms. Collins for her support of the war with Iran — what he calls a “new forever war” — and her “weak condemnation” of the 2022 Supreme Court decision to overturn Roe v. Wade.

The spot intercuts footage of a speech Mr. Platner gave in Portland with grainy footage of Ms. Collins speaking on the Senate floor. It also includes shots of a younger Mr. Trump joking with Jeffrey Epstein — another sign that Democratic candidates this year may try to capitalize politically on Republican voters’ anger about the deceased sexual predator.

“We don’t care that you are concerned while we go broke as you sell us out to the president and to the Epstein class who are engineering the greatest redistribution of wealth from the working class to the ruling class in this nation’s history,” Mr. Platner says.

The ad is one of three spots released by the Platner campaign since Gov. Janet Mills announced her exit from the Democratic primary on April 30, making Mr. Platner the party’s presumptive nominee. The other two introduce Mr. Platner to voters as a small-town oysterman and Marine veteran and criticize Ms. Collins for not holding public town-hall events.

Ms. Colllins’s ad — the first released by her campaign — focuses on her power in Washington and the economic benefits she has brought to Maine thanks to her role on the Senate Appropriations Committee.

The ad recounts her work securing $6 million to help replace the Eastport Breakwater, a pier in Southern Maine that collapsed one night in 2014.

“When the breakwater collapsed, it brought a halt to most of our economics and brought a tremendous amount of uncertainty to our future,” says Chris Gardner, the executive director of the Eastport Port Authority. “But prior to sunrise, we received a phone call from Senator Susan Collins, who assured us that she would be there to help see it rebuilt.”

Notably, the ad shows Ms. Collins in only brief glimpses of video. On Wednesday, she disclosed that she had long had what she called a benign essential tremor. Her remarks came amid mounting online scrutiny — pushed in particular on the left — after Ms. Collins announced her re-election through a social media video in which she is visibly shaking.

The new ad is part of an effort by Ms. Collins and her allies to draw a sharp contrast between her long experience in Washington and Mr. Platner’s rocky personal history.

A spot released last week by a super PAC backing her bid resurrected inflammatory comments made online by Mr. Platner that were dismissive of rape victims and insulted rural people, among other things. It also mentioned a skull-and-crossbones tattoo Mr. Platner had that resembled a Nazi image. He has said he did not know the tattoo’s origin and had it covered up last fall.

Katie Glueck contributed reporting.