Biden talks about passing torch in address

Kamala Harris' presidential campaign expressed optimism about its path to the presidency this week, telling reporters in a memo that the change atop the ticket could help the vice president reach a new crop of voters as the campaign plans an aggressive approach to key battleground states.

The extraordinary and rapid ascension of Harris to be the presumptive Democratic nominee "opens up persuadable voters," campaign manager Jennifer O'Malley Dillon wrote.

"The race is more fluid now. The Vice President is well-known but less well-known than both Trump and President Biden, particularly among Dem-leaning constituencies," she added.

O'Malley Dillon said some in this "expanded universe of winnable voters were previously Democrats and support down-ballot Democrats this cycle," a possible reference to the fact that some polls showed Biden with less support than Democrats running further down the ballot.

The memo outlined a plan to capitalize on Harris' appeal with Black, Latino and women voters. It also suggested Harris could attract people who did not vote for the Democratic ticket in 2020 but moved toward Democrats in the years since.

Meanwhile, O'Malley Dillon stressed that the campaign has "multiple pathways" to 270 electoral votes and plans to "play offense" in the competitive Blue Wall states of Michigan, Wisconsin and Pennsylvania, as well as the Sun Belt states of North Carolina, Georgia, Arizona and Nevada.

Since Sunday, Harris has earned the backing of Democratic Party leaders and enough Democratic National Convention delegates to make her the nominee if they kept true to their pledges -- a major milestone for the vice president.

The memo comes a day after the vice president had her first campaign event in battleground Wisconsin on Tuesday, sharply framing her race against former President Donald Trump.

Former Secretary of State Hillary Clinton published an op-ed in the New York Times on Tuesday reaffirming her support of Harris and warning of the attacks that Republicans will use against her during the campaign.

The 2016 Democratic nominee said that Harris is "chronically underestimated, as are so many women in politics, but she is well prepared for this moment," citing the vice president's experience as a prosecutor, U.S. senator and four years in the White House.

"Harris has sat with the president in the Situation Room, helping make the hardest decisions a leader can make. And when the extremist Supreme Court overturned Roe v. Wade, she became the administration's most passionate and effective advocate for restoring women's reproductive rights," Clinton wrote.

Clinton referred to her own candidacy and the sexist attacks labeled against her by Republicans and critics. She said Harris "will face unique additional challenges as the first Black and South Asian woman to be at the top of a major party's ticket."

"Ms. Harris's record and character will be distorted and disparaged by a flood of disinformation and the kind of ugly prejudice we're already hearing from MAGA mouthpieces. She and the campaign will have to cut through the noise, and all of us as voters must be thoughtful about what we read, believe and share," Clinton said.

Arizona Sen. Mark Kelly dodged questions from an ABC News reporter Tuesday about whether he received vetting materials for a potential vice presidential ticket.

"This is not about me," he said. "I'm going to be focused on doing everything I can to make sure she is elected because we cannot have a repeat of what we saw between 2016 and 2020."

Kelly is seen as one of the front-runners to be Vice President Harris' running mate, sources have told ABC News.

MORE | Who is Sen. Mark Kelly, a possible Harris VP pick?

Harris is scheduled to visit Houston on Thursday to deliver the keynote speech at the American Federation of Teachers' 88th national convention, according to the campaign.

"The Vice President's speech to educators from communities throughout the nation is a continuation of her consistent efforts to fight for workers across America, including as Chair of the White House Task Force on Worker Organizing and Empowerment," the campaign said in a statement.