WASHINGTON — Cory A. Booker, who gained celebrity as a danger-defying, super-tweeting mayor of Newark, was sworn in as New Jersey’s junior United States senator on Thursday. He is the first African-American to be elected to the chamber since Barack Obama in 2004.

Mr. Booker’s arrival in Washington did not come with the same political portent rendered by another high-profile senator who arrived here via a special election — Scott Brown, Republican of Massachusetts, whose 2010 victory signaled the ensuing power of the Tea Party movement, cost Democrats a 60-vote supermajority and placed a Republican in the seat held for nearly half a century by Edward M. Kennedy. (Senator Brown lost the seat to Elizabeth Warren in 2012, thus ending his swing-vote-laden tenure.)

But Senator Booker was met with a fair amount of attention from his fellow Democrats, whose excitement seemed to stem less from the fact that, after Senator Frank R. Lautenberg died in June, their party retained the seat as expected — but rather at his significant national star wattage and the fund-raising potential it may bring. He joins Senator Tim Scott of South Carolina, who was appointed by Gov. Nikki Haley earlier this year to replace Jim DeMint, who retired, as one of only two black senators.

Mr. Booker began his day, he said, with his mother, Carolyn Booker, in the office of Representative John Lewis, Democrat of Georgia, the civil rights leader. “It was really emotional and moving,” he said.