Biden campaign is polling on Harris' strength against Trump: Source

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, as a growing number Democrats are calling on him to step aside.

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

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"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."

The New York Times was first to report on the campaign's actions.

A new ABC News/Washington Post/Ipsos poll found Biden continues to run evenly with Trump: Americans were divided 46-47% between Biden and Trump if the election were today.

But two-thirds of Americans, including a majority of Biden's supporters, said he should step down as his party's presumptive nominee. On the issue of age, 85% of respondents said Biden he is too old for a second term, up a few points from April.

Were Harris to replace Biden as the Democratic nominee, the poll found Harris leading Trump 49-46% among all adults and 49-47% among registered voters.

MORE: Biden and Trump tied despite debate, as 67% call for president to drop out: POLL

Harris recently defended Biden as she's hit the campaign trail.

"Now, we always knew this election would be tough," she said on Tuesday at as the campaign launched an outreach effort to Asian American voters. "And the past few days have been a reminder that running for president of the United States is never easy."

"But the one thing we know about our President Joe Biden, is that he is a fighter, and he is the first to say, 'when you get knocked down you get back up,'" Harris said.

ABC News' Alexandra Hutzler contributed to this report.

Biden campaign is polling on Harris' strength against Trump: Source originally appeared on abcnews.go.com