Mr. Trump, for his part, has invested a huge amount of political capital in Pennsylvania this year.
He personally pushed for Dr. Oz to run for Senate, which ignited a nasty, three-way primary. If Dr. Oz loses on Tuesday, it would raise further questions about Mr. Trump’s ability to win the state in 2024 after he lost it in 2020.
The former president’s closing message this year has focused on his portrayal of an America in decline, with Democrats as the leading cause.
“We have to basically, in a nutshell, save our country,” Mr. Trump said Wednesday in Iowa.
In Pennsylvania, Mr. Trump’s super PAC, MAGA Inc., has targeted voters in the final days of the race with short digital ads, sent via text and web-embedded digital ads, that feature clips of Dr. Oz and Mr. Trump together at rallies in the state, according to videos reviewed by The New York Times.
The super PAC has sent similar videos of Trump-endorsed candidates to voters in Arizona, Ohio, Michigan and Nevada.
For Mr. Trump and Mr. Biden, the stakes could extend well beyond this year’s fight for control of Congress.
Already, both men have taken early steps toward announcing another run for president in 2024.
Mr. Trump has relentlessly teased supporters about a third White House campaign, and is expected to formally announce another bid soon after the midterms.
Mr. Biden has suggested that he would be more likely to run if Mr. Trump enters the race, with people close to the president saying they believe he is the Democrat best positioned to defeat his former opponent. Still, Mr. Biden, who will turn 80 this month, has shown signs of political weakness, even among his base.