(CNN) - Sen. Frank Lautenberg didn't get the best birthday gift on Wednesday, as new poll numbers indicate that even though a majority of New Jersey voters approve of the job he's doing in Congress, they don't think he deserves re-election next year.
Meanwhile, fellow Democrat and Newark Mayor Cory Booker continues to lead the now 89-year-old senator in a hypothetical 2014 Senate Democratic primary match-up.
Forty-five percent of registered voters in the state said they don't think Lautenberg should take on another term, while 36% disagree, according to the new Quinnipiac University poll. And though he has an approval rating of 50%-34% and a positive favorability rating, nearly three-fourths of voters say his age makes the work too difficult.
Booker, who's exploring a senatorial bid, has a higher likability score among every group measure, the poll shows. Earlier this month he registered the "Cory Booker for Senate" committee with the Federal Election Commission, which allows him to raise money for his campaign for the seat.
Booker leads the incumbent in a possible Democratic primary by a margin of 51% to 30%, according to the Qunnipiac poll.
A Fairleigh Dickinson University PublicMind poll released earlier this month indicated that 42% of self-identified Democrats and those who lean Democratic say they would prefer the two-term mayor, who's considered a rising star in the party, as the Democratic nominee, with 20% saying they would prefer Lautenberg.
While Lautenberg has yet to say whether or not he'll run next year, the senator made headlines Tuesday when he hit back at Booker for expressing intent to challenge him in a Democratic primary.
"I have four children, I love each one of them. I can't tell you that one of them wasn't occasionally disrespectful, so I gave them a spanking and everything was OK," Lautenberg told reporters.
It was Lautenberg's first comment on Booker's candidacy since Lautenberg's spokesman, Caley Gray, said earlier this month a report that "Senator Lautenberg has decided to retire is simply not true."
Quinnipiac University surveyed 1,647 registered voters in New Jersey by telephone from January 15-January 21. The poll's overall sampling error is plus or minus 2.4 percentage points. The survey includes 616 Democrats with a sampling error of plus or minus four percentage points.
– CNN's Dana Bash, Gregory Wallace, and Paul Steinhauser contributed to this report.