Sen. Frank Lautenberg's (D-N.J.) decision not to seek reelection in 2014 leaves Newark Mayor Cory Booker (D) as the favorite to succeed him as senator.

But Booker is unlikely to get a free ride.

Rep. Frank Pallone (D-N.J.).

A New Jersey Democrat with knowledge of Rep. Frank Pallone's (D-N.J.) plans tells The Fix that Pallone will run for the seat now that Lautenberg is out, setting up a test for Booker in the Democratic primary.

"Frank Pallone is running," the Democrat said.

Pallone may not be the only Democrat to look at the race. Despite his all-star image, Booker has increasingly come under scrutiny in the Garden State, including in a recent New York Times story that suggested his results as mayor didn't line up with his media profile.

Monmouth University poll Thursday shows just 38 percent of New Jersey Democratic primary voters think their party should instantly line up behind Booker, while 31 percent say other candidates should run and 23 percent had no opinion.

In addition, there are lots of ambitious Democrats in the strongly blue Garden State who have been waiting for an opportunity to run statewide. Among them is state Senate President Steve Sweeney.

But smart Democrats in the Garden State say they expect the race to come down to Booker and Pallone, and the onus will be on Pallone to compete with the frontrunner.

"Feels like a Booker versus Pallone showdown with everything else in the second tier," said one Democratic strategist from New Jersey.

The Democrat close to Pallone acknowledged that Booker is the favorite but said he won't have a cakewalk.

"It’s not that he’s not the likely senator, but he’s not the presumptive senator," the Democrat said.

Pallone is expected to try to run to Booker's left, noting the mayor's defense of Mitt Romney's work in venture capital, his buddy-buddy relationship with Republican Gov. Chris Christie and his hesitation to embrace an assault weapons ban.

In the same Monmouth University poll, Pallone trailed Booker and Lautenberg badly in the primary, taking just 4 percent to Booker's 40 percent and Lautenberg's 25 percent.

But much of that is owed to the fact that Pallone is not well-known outside his district. In addition, Democratic primaries in New Jersey depend very much on who gets the endorsement of the respective county parties (these endorsements are noted on the ballot and tend to play a big role in who wins in those counties).

Booker, while a national media star, hasn't built huge inroads with these local Democratic groups, which could leave Pallone an avenue to beat Booker in the ballot game. Pallone has been in his seat since 1988 and was a local politician before that, so he's built up plenty of relationships over the years.

Of course, that might not matter if Booker can continue to look like the political phenom he is reputed to be.

But nobody should be crowning Booker as the next senator just yet.