Donald Trump needs a vibe shift.

In the week since President Biden dropped his re-election bid and endorsed Vice President Kamala Harris , the expected Democratic nominee has largely erased the Republican nominee’s edge in polling and fundraising. And Trump has seen his vice-presidential pick, JD Vance, privately second-guessed by fellow Republicans who worry the Ohio senator isn’t the right candidate to win over swing voters in a race that has become much closer than it was when he was chosen.

Now, 100 days from the election, Trump is trying to shift the spotlight to what Republicans see as Harris’s weaknesses to regain the momentum he built during the GOP convention after an assassination attempt on his life.

Trump in events over the weekend stepped up verbal attacks on Harris, portraying her as an ultraliberal who is soft on crime and immigration. Trump also complained about media attention surrounding Harris, saying his opponent was being portrayed like former U.K. Prime Minister Margaret Thatcher. He also accused Democrats of staging a “coup” to remove Biden, an idea Democrats have dismissed as ridiculous.

“They took him out of the fight, and they put in a new fighter. We have a new victim now, Kamala,” Trump said Saturday night in St. Cloud, Minn. “We have a brand-new victim. And honestly, she’s a radical left-wing person.”

Trump’s rallies continue to draw large, energetic audiences and he has accelerated calls for them to show up and vote, suggesting a massive turnout would make the election “too big to rig.” But they are also forums for him to improvise, providing fodder for opponents. Appearing Friday before a group of religious activists in West Palm Beach, Fla., the former president said Christians as a whole don’t exercise their political power and implored them to participate.

“You won’t have to do it anymore. Four more years, you know what, it will be fixed, it will be fine. You won’t have to vote anymore, my beautiful Christians,” Trump said. The Harris campaign cited the remark to say Trump had promised to “end democracy.”

Republican Sen. Tom Cotton of Arkansas, speaking on CNN, called the statement a joke about how bad things have been for voters under Biden and how good they would be if Trump were re-elected. Kate Bedingfield , Biden’s former communications director, said on X she thought Trump’s message meant: “I won’t be on the ticket either way, so who cares.”

Trump allies say any momentum shift is being overblown by the news media and they expected Harris to get a bounce. “But the fundamentals of the race stay the same,” Trump campaign pollster Tony Fabrizio said in a memo last week, adding that soon, “Harris’ ‘honeymoon’ will end and voters will refocus on her role as Biden’s partner and co-pilot.”

Harris has sought to temper expectations, telling an audience of about 800 donors at a fundraiser in Pittsfield, Mass., on Saturday that Democrats remained the underdogs in the race but she pointed to the momentum of the first week of her campaign.

The vice president also sought to needle the Trump-Vance ticket, telling her supporters that Trump was “resorting to some wild lies about my record and some of what he and his running mate are saying is just plain weird. I mean, that’s the box you put that in.”

Minnesota Gov. Tim Walz , a Democrat under consideration to become Harris’s running mate, helped coin the term earlier in the week, calling the ticket on MSNBC, “just weird—they’re running for He-Man Women-Haters Club.”

Responding to reports that some Republicans say Trump made the wrong choice for running mate, Senate Majority Leader Chuck Schumer (D., N.Y.) trolled the former president on Sunday by saying on CBS ’s “Face the Nation” that time was running out for him to replace Vance on the ticket—though there is no sign Trump is considering that.

On the same CBS show, Republican Sen. Lindsey Graham of South Carolina chuckled at characterizations of Vance as weird and said Trump doesn’t regret adding him to the ticket.

“No, not at all, because JD Vance has one of the most compelling stories in American politics,” he said.

Trump advisers maintain that Vance is a good fit for the campaign and pointed to his more aggressive showing on Saturday in Minnesota, where he accused Harris of helping cover up Biden’s condition.

The Harris campaign said Sunday it raised $200 million in less than a week, after Biden’s exit from the race and endorsement of her. Roughly two-thirds of that money came from first-time donors, and more than 170,000 new volunteers have also signed on, the campaign said.

The money collected by Harris in a week is significantly more than the $127 million the Biden campaign said it raised with the Democratic Party for all of June, or the $112 million Trump and the GOP said they pulled in that month.

The race to define Harris to voters accelerated over the weekend, with super PACs on both sides unveiling new television ads in swing states.

A series of battleground-state surveys from Fox News showed the two candidates roughly tied in the “blue wall” states—Michigan, Wisconsin and Pennsylvania—seen as Democrats’ best path to keeping the White House.

A new Wall Street Journal national poll that showed the race between Harris and Trump within the margin of error also revealed 52% of voters view Harris unfavorably and 46% see her in a positive light. Millions will likely be spent by both sides in the coming weeks to try to move those numbers, because views about her are likely less rigid than those surrounding Trump, a longtime candidate and former president.

MAGA Inc., the top super PAC backing Trump, started an ad that calls Harris a “dangerous San Francisco liberal.” Another spot from the group focuses on a “chaotic border mess,” as it reminds viewers Biden asked her to focus on problems at the southern border.

Those two ads followed one that MAGA Inc. started airing shortly after Harris announced her candidacy. It claims she “covered-up Joe’s obvious mental decline.”

In the week since Harris entered the race, MAGA Inc. has spent $6.5 million on television, radio and digital ads that have run most heavily in Arizona, Pennsylvania and Georgia, according to AdImpact data.

Preserve America, a super PAC heavily financed by casino billionaire Miriam Adelson , has started attacking Harris in Wisconsin, Michigan and Pennsylvania. The group has spent about $2.4 million in those states since Friday on an ad that focuses on the vice president’s role with problems at the southern border.

Harris has yet to broadcast any of her own ads, although a super PAC backing her did start to provide some air cover over the weekend. Future Forward started running a spot supportive of her on Saturday and has said it plans to spend $50 million during the next three weeks leading up to the start of the Democratic National Convention in Chicago.

Like those attacking her, the positive spot features scenes from San Francisco. “She’s the district attorney who protected children from sexual predators,” it says. “She’s the attorney general who stood up to the big banks to protect homeowners.”

The Future Forward ad is running in Arizona, Georgia, Michigan, Nevada, Pennsylvania and Wisconsin. The group has spent $1.6 million on Harris’s behalf since Saturday.

Democrats are also urging Harris to keep promoting a forward-looking agenda that lays out the priorities of her potential administration, something that the campaign is working on.

“The argument, I think, has been well honed about what’s wrong with Donald Trump,” said former Massachusetts Gov. Deval Patrick , a Democrat who helped organize Harris’s fundraiser. “But there’s more to be said about what we want to do the day after we beat Donald Trump, what are the next four years and beyond about?”

While the race has shifted, Trump has made clear to supporters that he remains the same after the assassination attempt. In the immediate aftermath he and his campaign stressed a message of national unity.

“They all say, ‘I think he’s changed. I think he’s changed since two weeks ago. Something affected him,’ ” he said Saturday in Minnesota. “No, I haven’t changed. Maybe I’ve gotten worse. Because I get angry at the incompetence that I witness every single day.”

Write to John McCormick at mccormick.john@wsj.com , Ken Thomas at ken.thomas@wsj.com , Alex Leary at alex.leary@wsj.com and Elizabeth Findell at elizabeth.findell@wsj.com