On Wednesday, Graham Platner buckled on a helmet and rode into the general election.
Hours after being anointed as the Democratic nominee for Senate in Maine, Mr. Platner posted a video of himself riding a bike with a Bar Harbor community group and delivering a message of national unity.
“Everybody really just wants to help each other out,” he said. “It’s the message that we need to take into our politics.”
But in Washington, signs of how divisive the Maine general election will become quickly emerged. Both parties began to lay out their lines of attack in a contest that both sides have described as central to their midterm strategies.
President Trump used an appearance in the Oval Office to deliver a two-minute monologue attacking Mr. Platner, calling him a “thug” nearly a dozen times and assailing Democratic leaders for supporting his bid.
“He’s worse than any human being that’s ever run for office, probably,” Mr. Trump said. “He’s a cheap, no-good person.”
The president offered tepid praise for Senator Susan Collins, the moderate Republican whom Mr. Platner is trying to unseat, saying that while “she’s not my best friend at all,” she was “a sane person.”
Facing a difficult national political environment, Republicans are hoping to transform the contest into a referendum on Mr. Platner’s character and his liberal positions on issues like universal child care, “Medicare for all” and immigration enforcement.
They are warning donors that Mr. Platner remains a strong contender for the seat, even after a series of scandals over his past conduct and statements that have made some Democrats nervous about his candidacy.
“The political fundamentals in Maine remain challenging, and it is a fatal mistake to assume Platner is too damaged to win,” the Republican Senate campaign arm wrote in a memo released on Wednesday morning. “National donors and supporters should not mistake Democrat discomfort for Democrat retreat.”
In the hours after Mr. Platner’s primary victory, Democratic leaders in Washington appeared to grudgingly accept his candidacy, offering restrained support for his bid.
Senator Chuck Schumer, the minority leader, and Senator Kirsten Gillibrand of New York, the chair of the Democratic Senate campaign arm, released a joint statement of support that predicted that Mr. Platner would defeat Ms. Collins, though it included no words of praise for the party’s nominee or his primary campaign. The Senate leaders’ preferred candidate, Gov. Janet Mills of Maine, suspended her bid in April after struggling to match Mr. Platner’s fund-raising and voter enthusiasm.
Their statement, which focused on Ms. Collins, offered a preview of their general-election strategy of making the contest about the incumbent’s five terms in the Senate and her support for the Trump administration.
“Platner’s agenda supports working people and families, while Collins upholds Washington’s status quo,” Lauren French, a spokeswoman for the main super PAC supporting Democratic Senate candidates, said in a statement.
Mr. Platner echoed that message in his early attacks, casting Ms. Collins as an “establishment politician who doesn’t really believe in anything” in an interview on Wednesday morning on MS NOW’s “Morning Joe.”
His first campaign ad of the general election focused on the ties between Jeffrey Epstein, the convicted sex offender, and both Republicans and Democrats.
“The only thing the party establishments can agree on is a love of Jeffrey Epstein and a hatred of me,” Mr. Platner said in the spot.
He has also highlighted Ms. Collins’s support for the 2018 Supreme Court confirmation of Justice Brett M. Kavanaugh, who later voted to overturn federal abortion rights. Asked by a CNN reporter on Wednesday whether she regretted her vote, Ms. Collins declined to respond.
“The people of Maine know me well, and I know them well,” she said in response to a previous question about Mr. Platner’s attacks.
Among Democrats, divisions — and concerns about Mr. Platner’s past — linger.
Throughout his primary campaign, Mr. Platner has been trailed by controversies, including a tattoo he had that resembled a Nazi symbol, a history of inflammatory online posts and reporting from The New York Times in which women he dated accused him of engaging in unsettling behavior.
Mr. Platner has pointed to his struggles with PTSD after his combat tours with the Marines. He has also denied allegations that he was physically intimidating to women. He has sought to raise doubts about the claims by noting that one woman who spoke with The Times has worked in conservative politics.
Senator Mark Warner of Virginia, a Democrat, urged Mr. Platner to take more aggressive steps to dispel the allegations.
“I do wish this, that if Mr. Platner feels that he could — and I’ve never met him — if he could disprove some of these accusations, I think that would be important,” he said in an interview on Wednesday on CNN.
In the nearly 30-minute interview on “Morning Joe,” Mr. Platner was repeatedly pressed about revelations regarding his past and concerns that explicit photos or text messages could be weaponized against him.
He said that such issues were largely irrelevant to voters in Maine.
“I just want to make this clear: There’s nothing out there that’s actually concerning,” he said. “People will make everything seem very concerning, because that’s what people do in politics.”
Recent revelations of explicit text messages he sent to women while he was married were being “blown out of proportion” by news outlets and his opponents, he said.
“Since this thing started, we have been told time and time again by supposed political geniuses who are professionals and really know what they’re doing, that the campaign was over,” said Mr. Platner, clad in a purple hooded sweatshirt.
He added: “We’ve been counted out time and time again. And then it doesn’t happen.”
Nick Corasaniti contributed reporting from New York.
