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 (D-N.J.), who officially jumped into the 2020 Democratic primary Friday, told fellow presidential candidate Sen. Kristen Gillibrand (D-N.Y.) he was glad they were both running for president.

Huge smile! Kirsten, I am glad we are both in.



As I told folks today on The View:



You are my sister. There may be some sibling rivalry but we will always be family. https://t.co/EHqfZtdKYN — Cory Booker (@CoryBooker) February 1, 2019

“You are my sister. There may be some sibling rivalry but we will always be family,” he tweeted Friday.

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The comments came in response to a tweet from Gillibrand saying she would be cheering Booker on, just “not TOO hard.”

The two senators are known to be good friends.

Booker and Gillibrand are only two of nine people who have already said they are running or are leaning toward making a bid. The primary field is expected to be the largest in modern history and could include upwards of eight senators alone.

Along with Gillibrand, Sens. Elizabeth Warren Elizabeth WarrenJudd Gregg: The Kamala threat — the Californiaization of America GOP set to release controversial Biden report Biden's fiscal program: What is the likely market impact? MORE (D-Mass.) and Kamala Harris Kamala HarrisJoe Biden looks to expand election battleground into Trump country Fox's Napolitano: Supreme Court confirmation hearings will be 'World War III of political battles' Rush Limbaugh encourages Senate to skip hearings for Trump's SCOTUS nominee MORE (D-Calif.) are also in the race.

It is unclear how long the niceties can last between the senators running in 2020, as many will likely be angling for the approval of the progressive wing of the party’s base.