Booker will have to raise money for himself, too. He will be up for re-election in November 2014 and is likely to face more competition. Menendez said he believes many more Republicans will want to run in a traditional election year rather than during a special election.
"He'll likely have a more substantial challenger next time," Menendez said. Booker beat Lonegan by 11 percentage points.
The time he'll spend campaigning and fundraising makes one of his new office's most prized commodities — time in the Senate — more difficult to come by.
"For the next year he's going to be on a bit of a marathon as he both gets his office up, represents the people of the state, picks a couple of issues he thinks are important to New Jerseyans and runs for re-election," Menendez said.
President Barack Obama said he's happy Booker is heading south. Booker said the two are friends.
"He's going to do a great job," Obama said.
Booker said he's ready to take on a fractured Washington.
"I'm going into the Senate," he said Thursday on WNYW's "Good Day New York." ''If I break dishes on the way to try to serve the people of New Jersey, so be it."
Associated Press writer Darlene Superville in Washington contributed. Follow Zezima at www.twitter.com/katiezez