MIDDLETOWN, Ohio — A week after Ohio Sen. J.D. Vance became Donald Trump’s running mate, set to take on President Biden’s running mate Kamala D. Harris, he faced an unexpected challenge: Harris is likely to top the Democratic ticket and her vice-presidential pick is unknown.

So in his first solo rally, Vance instead took aim Monday at the Democratic Party as a whole, claiming without evidence that Biden was pushed out by a group of party elites and accusing Democrats of being anti-democratic given the president’s announcement Sunday that he would withdraw as their presumptive nominee. Vance suggested the decision was made by former president Barack Obama and Democratic megadonor George Soros, two figures often cast as boogeymen by the far right.

“The idea of selecting the Democrat party’s nominee, because George Soros and Barack Obama and a couple of elite Democrats got in a smoke-filled room and decided to throw Joe Biden overboard, that is not how it works,” Vance said to cheers from hundreds of supporters gathered at his alma mater high school. “That’s a threat to democracy.”

His audience nodded along with his attacks, applauding when he said the vice president was unpatriotic because she hadn’t expressed enough gratitude for the country.

The Harris team quickly retorted by pointing to the Trump campaign’s billionaire donors and hinting at Vance’s connections to Silicon Valley as a former venture capitalist.

“That’s rich coming from extremist JD Vance, who is bought and paid for by Elon Musk and Silicon Valley and has promised to raise taxes on working families and give handouts to corporations and billionaires,” said Harris campaign spokesman Joseph Costello.

The attacks offered the first glimpse of Vance workshopping his role on the ticket, to be deployed by the GOP to appeal to Republican voters and disparage the Democratic Party given its weeks of hand-wringing and rancor after Biden’s disastrous performance in the first presidential debate.

Vance, a freshman senator, would be the least experienced vice president in decades if elected. But voters in attendance here were unconcerned about his ability to take on whomever the Democrats choose as their vice-presidential candidate. Die-hard Trump fans in attendance said they were confident now more than ever, a week after Trump was wounded at a rally in Butler, Pa., that the electorate would overwhelmingly choose the Republican ticket.

“It doesn’t matter,” retired social worker Leona Rader, 71, of New Carlisle, Ohio, said of Democrats’ unknown running mate. “Trump and Vance are going to win. Vance is a check mark for Trump.”

Rader and others remarked on the senator’s eloquence and predicted he could face off against any potential challenge in a debate. In his deliberations for his running mate, Trump considered Vance’s ability to defend him in TV interviews a major plus.

Yet outside Middletown High School, local Democrats were abuzz with excitement for Harris as their likely presidential candidate. Cleveland Canova, the Democratic candidate for a nearby statehouse seat, stood with a sign reading “TRUMP KILLED ROE V WADE” and said Biden’s decision to step aside will likely help the party and other down-ballot Democrats like him.

“It was a big energy boost for the whole party,” he said.

Vance is expected to travel extensively in the coming months, speaking in battleground states about his hardscrabble background growing up in Ohio with family members who suffered from poverty and drug addiction, as he documented in his bestseller “Hillbilly Elegy.”

And he hinted at his excitement to take on Democrats, saying he was “pissed off” he couldn’t debate Harris. “I was told I was going to get to debate Kamala Harris, and now President Trump’s going to get to debate her,” he said to laughter from the crowd.

After Vance was tapped as Trump’s running mate last week, Harris called the Ohio lawmaker and encouraged him to pick a date for their debate. The Trump campaign responded that it couldn’t lock in a date until the Democratic ticket is set.

“To do so would be unfair to Gavin Newsom, J.B. Pritzker, Gretchen Whitmer, or whoever Kamala Harris picks as her running mate,” Trump campaign senior adviser Brian Hughes said in a statement.