Donald J. Trump’s campaign is preparing a major effort to attack Vice President Kamala Harris if President Biden steps aside as the Democratic nominee, including a wave of ads focusing on her record in her current office and in California, according to two people briefed on the matter.

The Trump team has already prepared opposition research books on Ms. Harris, and has similar dossiers on other Democrats who could become the nominee if Mr. Biden were to drop out of the race.

But the bulk of the preparations so far have been focused on Ms. Harris, including a recently concluded poll testing her vulnerabilities in a general election contest, according to the two people. The Trump team’s attention on Ms. Harris is based on its assumption that if Democrats were to bypass the first Black woman to serve as vice president, it would drive even deeper divisions in the party and risk alienating their base of Black voters.

Trump allies have also begun examining the records of Democratic governors who are considered potential running mates for Ms. Harris. Advisers to the former president are paying especially close attention to Gov. Josh Shapiro of Pennsylvania — the state the Trump campaign is most focused on winning to block the Democrats’ path to the White House.

A Trump campaign spokesman did not respond to an email seeking comment.

Brian Fallon, a campaign spokesman for Ms. Harris, said in a statement: “After tanking the bipartisan border deal, Donald Trump has resorted to lying about the vice president’s record. As a former district attorney and attorney general, she has stood up to fraudsters and felons like Trump her entire career. Trump’s lies won’t stop her from continuing to prosecute the case against him on the biggest issues in this race.”