By Thomas Ferraro

WASHINGTON (Reuters) - President Barack Obama's fellow Democrats welcomed a new colleague to the Senate on Thursday, newly elected Cory Booker, and the additional vote Booker gives them in the Senate.

Vice President Joe Biden administered the oath to Booker, 44, elected this month as the first black senator from New Jersey. He resigned this week as mayor of Newark, the state's largest city.

A Rhodes scholar and Yale Law School graduate who is widely seen as a rising political star, Booker fills the Senate seat vacated by the death in June of fellow Democrat Frank Lautenberg, who was 89.

Lautenberg's death, coupled with New Jersey Governor Chris Christie temporarily replacing the late senator with a Republican, cut Democrats' control of the Senate to 54-46.

Booker's arrival puts Democrats' hold on the Senate back at 55-45. And it puts them a bit closer to the 60 votes that are routinely needed to clear Republican procedural roadblocks against much of the president's agenda.

Minutes after being sworn in, Booker cast his first vote - to end one of these roadblocks against Obama's nomination of Melvin Watt as director of the Federal Housing Finance Agency.

Democrats fell short, 56-42.

Booker joined Tim Scott of South Carolina as the nation's only two black senators. Scott walked up to Booker after the swearing in ceremony and shook his hand.

Scott was appointed to the Senate earlier this year to fill the vacancy created by the surprise retirement of Jim DeMint, a fellow Republican.

Scott is expected to win a full six-year term next year. If he does, Scott and Booker will make history as the first two elected black senators serving at the same time, according to the Senate historian's office.

Booker arranged to meet later in the day with a Obama, a former U.S. senator from Illinois who backed Booker's Senate bid.

Before being sworn in, Booker met privately with Senate Majority Leader Harry Reid, a Nevada Democrat.

Afterward, Reid praised Booker, who served the past decade as a Newark city councilman and then mayor. He focused on improving life in the inner city.

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"I urge my fellow senators, Democrats and Republicans, to get to know this good man," said Reid, standing on the Senate floor. "We are fortunate to have him here."

(Reporting by Thomas Ferraro; Editing by Doina Chiacu)

(This story was refiled to drop extraneous word in the 11th graf)