WASHINGTON — U.S. Sen. Cory Booker sent out a plea for campaign cash shortly after Joe Biden entered the race for the 2020 Democratic presidential nomination, saying the former vice president’s announcement “poses a real challenge" to his own candidacy.

“The truth is this poses a real challenge for an underdog campaign like ours,”Booker, D-N.J., wrote in the fundraising email.

Biden is the 22nd Democrat to enter the race, and Booker’s email noted that the Democratic National Committee will choose participants for its debates based on how many donors each candidate has.

The first debate is scheduled over two days in June in Miami.

Biden leads the Democratic field in the Real Clear Politics polling average, with 29.3 percent. Booker is in seventh place with 3.5 percent.

Booker apparently has qualified based on his standing in opinion polls, while lagging behind many of his opponents in the number of individual donors.

“Space on the debate stage is limited, and we know the DNC will decide who makes it based in part on how many donors each campaign has,” Booker wrote. “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.”

Booker, with ties to Wall Street, Silicon Valley and the pro-Israel community, took in 55 percent of his campaign cash in donations of more than $200, according to the Center for Responsive Politics, a Washington-based research group.

By contrast, U.S. Sen. Bernie Sanders of Vermont, who raised more money than any other Democratic presidential candidate from January to March, brought in 14 percent in large contributions.

Overall, Booker reported raising $7.9 million, including $5.1 million in contributions and $2.7 million transferred from his Senate campaign account.

Jonathan D. Salant may be reached at jsalant@njadvancemedia.com. Follow him on Twitter @JDSalant or on Facebook. Find NJ.com Politics on Facebook.

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