Their campaign has focused on the gaffe-prone president’s age and mental fitness for the job, but now Trump is the older candidate and could face the same questions.
Conservative political action committees (PACs) have spent millions of dollars on advertising levelled at Mr Biden, which will now go to waste.
Trump-allied PACs are already preparing attack ads targeting Ms Harris, a 59-year-old black woman who could now become the first female president.
They plan to paint her as a puppet master in the Biden administration in the ads, which include a $5 million TV spot in the battleground states of Pennsylvania, Georgia and Arizona.
Presidential battle lines
Ms Harris’ campaign has already contrasted her past work as a prosecutor unfavourably with Trump’s recent criminal convictions for covering up hush money payments to the porn star Stormy Daniels.
She is vocal on abortion rights, which Trump views as a liability at the ballot box after Republicans underperformed in the 2022 midterms.
Those elections were held after the conservative-controlled Supreme Court overturned Roe vs Wade, which guaranteed a constitutional right to abortion.
Ms Harris has said she wants to “earn and win” the nomination but has not yet been endorsed by influential Democrats such as Barack Obama, the former president, or Nancy Pelosi, the former speaker of the House.
She has not yet been confirmed as the candidate or named a vice-presidential pick, which will further complicate Republican wargaming for the campaign.
“I will do everything in my power to unite the Democratic Party – and unite our nation – to defeat Donald Trump,” Ms Harris said shortly after Mr Biden’s dramatic exit.