Vice President Kamala Harris has tapped Kamau M. Marshall – formerly a senior adviser and spokesperson for the Biden-Harris ticket – as a 2024 senior adviser for her new presidential campaign.

Marshall, who led then-candidate Joe Biden's strategic communications during his 2020 bid for the presidency, started as a senior adviser for the Biden-Harris campaign earlier this month after more than a year as a senior adviser under Secretary of Education Miguel Cardona.

Marshall said now is the time for unity with just over a 100 days until the 2024 election.

"We often say that each election is the most critical of our lifetime, but this one is at all costs," Marshall told ABC News.

"I cannot emphasize the urgency of the upcoming election and how many achievements could be undone, rolling back all of the progress the Biden-Harris administration has made," he added.

Vice President Harris traveled to Milwaukee, Wisconsin, on Tuesday in the first battleground state rally of her campaign.

In a sudden reversal, President Biden, who had vowed for weeks that he would be staying in the presidential race, dropped out and endorsed Vice President Harris on Sunday.

"I won't be on a ticket, but I'm still going to be fully, fully engaged," Biden said on a call with senior campaign staffers on Monday evening.

A career communications strategist, the move made him one of the only Black men to join the campaign's senior-level staff.

Biden congressional ally Rep. Jasmine Crockett, D-Texas, celebrated the Harris ticket for transitioning to youth.

"He's younger, he's Black and he's male," Rep. Crockett said about Marshall, who is in his mid-30s. "Regardless of who's at the top of the ticket, I think it's really important that we tap into and we listen to young voices, Black voices, and definitely male voices. I think that he is someone who is able to tap into those cross sections."

In the wake of Monday's "Win with Black Men" live discussion -- a more than four-hour voter mobilization effort hosted via journalist Roland Martin's "Black Star Network" urging Black men to step up for Vice President Harris -- they have become a key demographic in this election cycle during which they have voiced their frustrations.

Meanwhile, more than 80% of Black men identified with the Democratic Party over the last 25 years, according to the Pew Research Center.

Some of the Black voters most likely to support former President Donald Trump are those under the age of 50, according to Pew. Others are considering staying home in November.

Democratic strategist and former Obama campaign adviser Ameshia Cross praised Marshall's experience with Black voters and his ability to reach Black men.

"He [Marshall] knows how to lead and engage those Black men who feel as though the party has left them or is not meeting their needs at this moment," Cross told ABC News.

Before joining Harris' campaign, Marshall, a former Houston Independent School District (HISD) teacher, championed several key education policies under Secretary Cardona's office of communications and outreach. He worked on the White House's initiative on HBCUs, historic HBCU funding, and college affordability.

However, the last year at the department has been mired by higher education woes, such as the U.S. Supreme Court effectively ending affirmative action and mounting challenges to Biden's student debt relief plans. (The initial plan was struck down by the Supreme Court last year.)