Biden campaign is polling on Harris' strength against Trump

President Joe Biden's campaign is testing head-to-head matchups of Vice President Kamala Harris against former President Donald Trump, a source familiar with the strategy told ABC News. It's a strategic shift that comes with increasing scrutiny on whether Biden should end his 2024 reelection campaign.

The source casts this as a response to the fact that Trump has begun to attack Harris in his public statements and speeches.

"Donald Trump shifted his stump speech. We'd be dumb not to adjust," the source told ABC News. "We obviously pay close attention to what he is saying."