The two Democratic congressmen challenging Newark Mayor Cory Booker in the party's New Jersey Senate primary have failed to make a dent in the front-runner's wide lead, according to a new poll of the race. 

With less than a month to go before the Aug. 13 primary, a new Monmouth University poll shows Booker has the support of 49 percent of likely Democratic voters. Rep. Frank Pallone Jr. trails nearly 40 points behind with 12 percent, and Rep. Rush Holt has 8 percent.

A fourth candidate, New Jersey Assembly Speaker Sheila Oliver, has 3 percent.

Slightly more than a quarter of Democratic primary voters, 28 percent, are undecided.

Booker has held steady near 50 percent in polls since the start of the Senate campaign in early June. 

The survey suggests many of the attacks that Booker's primary opponents have begun to launch against the Newark mayor aren't sticking. 

Booker's foes have sought to criticize him over his friendship with New Jersey Gov. Chris Christie (R). But the poll found Democratic primary voters are split on their opinion of the governor, raising questions about the effectiveness of the tactic. 

Booker's opponents have also begun to question his commitment to Democratic principles, but the poll seems to indicate the party's primary voters don't necessarily agree on what those values are. 

Overall, 41 percent see the policy positions of the Democratic candidates as largely similar — an indication that voters will vote based on who they know and like best, rather than on the issues.

That means Booker's greatest asset could be his personal "celebrity," which critics have tried to cast as a liability. Sixty-four percent of Democratic primary voters view Booker favorably, while about a quarter have no opinion of him and only 8 percent view him unfavorably.

Sixty-five percent of those polled say they don't have an opinion on Pallone, and 70 percent say the same of Holt. 

To date, Pallone's biggest coup has been winning the endorsement of the late Sen. Frank Lautenberg's (D-N.J.) family. But the endorsement has barely registered with voters, as only 28 percent are aware of it, and only 16 percent say it makes them more likely to support the congressman.

Booker entered the race with a higher profile than his three opponents, and has done more to raise it among New Jersey voters. He remains the only candidate on air, launching his third ad Monday while his opponents continue to release Web ads.

His opponents have been reluctant to attack him outright as well, allowing him to maintain sky-high favorables in the race. 

Though they've knocked his celebrity and associations with Christie, and Pallone suggested Booker could be too close to Wall Street, the attacks have been muted, and often the candidates avoid mentioning the front-runner by name.

The survey was conducted among 403 likely Democratic primary voters from July 11-14 and has a margin of error of 4.9 percentage points.

Christie named New Jersey attorney general Jeff Chiesa as interim senator for the seat following Lautenberg's death, but Democrats are favored to win it back in the October special election.