Sen. Cory Booker Cory Anthony BookerBipartisan praise pours in after Ginsburg's death DHS opens probe into allegations at Georgia ICE facility Democratic lawmakers call for an investigation into allegations of medical neglect at Georgia ICE facility MORE’s (D-N.J.) presidential campaign on Thursday sent a fundraising letter following former Vice President Joe Biden Joe BidenBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Democratic groups using Bloomberg money to launch M in Spanish language ads in Florida Harris faces pivotal moment with Supreme Court battle MORE’s official entry into the 2020 presidential race.

In the solicitation, Booker's campaign said that Biden's entry makes the race even more difficult “for an underdog campaign like ours.”

“More voices make us a stronger party, but even I admit it’s getting hard to keep track of how many Democrats are running for president,” the email from Booker’s campaign reads.

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The email goes on to point out the limited room that will be allotted for all the candidates on the debate stage.

The Democratic National Committee has said no more than 20 candidates will be allowed to attend debates, with only 10 on stage at one time. Candidates must poll at 1 percent or more in three national or early primary state polls or receive contributions from at least 65,000 donors in 20 states to qualify.

Booker has qualified on the polling front for most of April, hovering between 2 and 4 percent throughout the month, according to RealClearPolitics.

Booker has joined most 2020 Democratic contenders in refusing to accept campaign donations from corporate PACs.

Booker's campaign says Biden further swelling the ranks of Democratic contenders “poses a real challenge.”

“Some campaigns begin with a massive head start when it comes to fundraising, but I haven’t spent the last few years building a massive online donor base.”