Biden talks about passing torch in address

March For Our Lives, the youth-led organization dedicated to ending gun violence following the 2018 Parkland, Florida, high school shooting, will be endorsing Vice President Kamala Harris in the 2024 election -- the first time the organization has ever endorsed a political candidate, and an indicator of the mounting youth support for the vice president.

"As one of the largest youth-led movements in the nation, we are clear-eyed about the challenge ahead, and we believe that Kamala Harris is uniquely suited to meet this moment," the group said in a news release shared first with ABC News.

The group goes on to say that Harris is the right candidate to meet the political moment the country currently finds itself in.

"We need an ardent defender of democracy, a gun violence prevention champion, and a leader who will listen to young people, give us a seat at the table, and fight for our future. We believe that Kamala Harris is that candidate and the right person to stand up for us and fight for the country we deserve," the news release read.

The organization's endorsement of Harris comes as she oversees the White House Office of Gun Violence Prevention and has met regularly with advocates and survivors of gun violence.

Natalie Fall, executive director of March For Our Lives, told ABC News that March For Our Lives will mobilize young voters to cast ballots for Harris and other down-ballot candidates through door-knocking, phone banking and creative campaigns.

The group's endorsement comes as youth voters and organizations mobilize behind Harris.

Voters of Tomorrow, a Gen Z-led organization that engages young Americans in politics and government, announced on Sunday their endorsement of Harris and their efforts to mobilize youth voters behind her.

Following their announcement, the group said it raised $125,000 on Sunday -- its best fundraising day ever.

"There's so much authentic excitement surrounding Vice President Harris online and on the ground, and now we're channeling that into political action," Jack Lobel, press secretary for Voters of Tomorrow, said to ABC News in an interview.

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?