Newly-elected Democratic senator from New Jersey, former Newark mayor Cory Booker, smiles in the Old Senate Chamber following an oath of office ceremony with Vice President Joe Biden. (J. Scott Applewhite/AP)

The newly sworn-in Sen. Cory Booker (D-N.J.) was still milling around accepting congratulations when his name was called for his first vote Thursday, and for just a second, he looked up in panic, like the kid who’s talking when the teacher calls on him and hasn’t heard the question.

Hesitantly, he raised his forefinger to signal his “aye’’ instead of giving the expected thumbs-up or thumbs-down, and when his friends in the visitor’s gallery erupted in laughter, he joined in, milking the moment by throwing his arms out wide, putting a hand over his heart and bowing ever so slightly, like a baseball star playing to the fans.

Even in a chamber crowded with big personalities, he was an instant standout. Some who know the D.C.-born, 44-year-old former mayor of Newark — marathon tweeter, corny-joke teller and news magnet, who in college talked a jumper off the ledge and last year pulled a neighbor from the second story of her burning home — swear that he’ll loathe this stodgy old club.

But his debut here said: Or, not.

The view of the Senate floor from the press gallery above is not the most flattering; not a few of the male domes below are covered with comb-overs. Only one sound stood out as members embraced, released and repeated — for a group that would rather fight than eat, this is one huggin’ bunch — and it was the new guy’s head-back cackle.

Even the few who were listening couldn’t hear Senate Majority Leader Harry Reid (D-Nev.) formally welcome his fellow Democrat because his mike wasn’t working. (For the record, he praised the filial devotion of “this fine young man’’ to the father who died earlier this month in Reid’s home state of Nevada after suffering a “very, very violent stroke.’’)

Just as Reid was wrapping up, the sound came on, and in case there’s any doubt that this is still a boys’ club, he noted that none of Booker’s many accomplishments as a Rhodes Scholar or Yale-educated lawyer or the rare mayor who actually lived in the inner city of the city he ran matched his moment on . . . the gridiron, of course: “For me, a frustrated, wannabe athlete, his most impressive qualification, as far as I’m concerned: He was a tight end for one of Stanford’s great football teams.”

Then the other biggest boy in the room, Vice President Biden, who The Post’s Michael Gerson suggested might be “the last genuine human being in American politics,” grinned and called Booker to come forward. “Come on up here, Frank,” Biden mistakenly directed the state’s other senator, Robert Menendez, before quickly correcting himself. Biden served for years with Booker’s long-serving predecessor, Frank Lautenberg, who died in June.

Biden whispered a couple of stage instructions — the left hand should go on the Bible, the same one Booker’s mother gave him for his swearing-in as mayor in 2006. The vice president then intoned the words that even he wouldn’t try to improvise or improve upon: “Do you solemnly swear that you will support and defend the Constitution of the United States against all enemies, foreign and domestic . . . and that you will well and faithfully discharge the duties of the office on which you are about to enter, so help you God?”

Booker swore his oath and was in. He is the first African American elected to the Senate since his friend Barack Obama took the oath in 2004 and only the fourth African American elected by popular vote in the nation’s history. South Carolina Republican Tim Scott, the chamber’s only other black senator, exchanged a manly double-arm pat with his new colleague. Scott was appointed to finish the term of Jim DeMint and is expected to run for the office next year, as is Booker.

After Booker finally tore himself away — or, rather, after Rep. Sheila Jackson Lee (D-Tex.) led him off the floor, civil rights icon Rep. John Lewis (D-Ga.) said in an interview that it had been a great and historic day: “To have another African American senator elected and now sworn in sends a strong message to our nation that we continue to make progress,’’ he said, but then sighed and added, “on the other hand, when we see what the president’s going through, it’s a reminder we still have a long way to go.”

Asked exactly what he was referring to, he put it this way: “Everything! While the president has achieved so much in his personal life, and so much in political life, there’s been a constant effort to block him from accomplishing almost anything; he could have achieved more if he’d gotten just a bit of cooperation. And there’s this effort to discount his achievements — even the fact that he was born in the United States.”

Booker’s own thoughts on the day were expressed via Twitter, of course, where he posted photos of himself with his smiling mother, Carolyn, and of the new nameplate on the door outside his office.

He’d been told, he tweeted, that he might be the Senate’s only vegetarian. And to a supporter who told him that the day had given him “a sliver of hope,’’ he tweeted back, “Never let finite disappointment undermine infinite hope.”

He’ll need all of that attitude he can get in a body that blocked two more of Obama’s nominees on Thursday.