Assembly Speaker Sheila Oliver touts herself as the choice for voters who want to see a woman added to New Jersey's all-male congressional delegation.
Booker was always expected to be the favorite in the race for the Democratic nomination, but New Jersey politicos saw a path to an upset if Pallone or Holt could galvanize liberal groups - labor unions, environmentalists, teachers; campaign from the left; and take advantage of the low turnout expected in an odd Aug. 13 primary, when a small but dedicated cluster of voters could tilt the race.
That strategy, though, depended on one of the congressmen taking on Booker one-on-one, standing out as the clear alternative for devoted Democrats who have chafed at the mayor's ties to Wall Street and Gov. Christie.
Instead, the two white liberal congressmen, with long stints in office and similar appeals to voters, may neutralize each other. Their districts abut in Central New Jersey, and they have voted together 97 percent of the time since 2007, according to the website OpenCongress.org.
"A lot of people would have a difficult time telling them apart in a lineup," said Matthew Hale, a political scientist at Seton Hall University. "They're going to steal each other's votes, it would seem."
The similarities have left each congressman struggling to break out from the pack and establish himself as the option worthy of liberal campaign money, grassroots aid, and votes. Along with trying to "dent" Booker, they are battling each other, Zelizer said.
"They're kind of doing a two-way competition," he said.
The tangled contest has paralyzed some groups that might have given Holt or Pallone an edge. Two of the state's most influential labor unions have been unable to pick between allies.
The New Jersey Education Association is not expected to choose sides, and the Communication Workers of America, the state's largest public employee union, has been unable to reach a decision.
"The thing I think our locals struggle most about is their relationship with the two congressmen. ... These are people who have never let us down," said Hetty Rosenstein, state director for the CWA, which represents 70,000 workers in New Jersey.
The unions' reach and organizational skills could have been potent weapons in the summer primary.
As Booker has racked up Democratic endorsements, Holt and Pallone have jousted over who is more active leader among two of the more left-leaning members of Congress.
Each has staked out a piece of the liberal spectrum.
Pallone, from the Shore, highlighted health care and the environment. His campaign logo features an ocean wave curling into the "O" in his name.
Holt's website labels him as a "teacher, scientist, progressive." The nuclear physicist has trumpeted his biography and work advocating for civil liberties and education, and against school vouchers, a direct contrast with Booker.
Pallone stressed that he helped write President Obama's health-care law and played a major role fighting for federal aid after Hurricane Sandy.
"You have to look and see who has spent the time getting the job done," Pallone said. "I have been very much out front in terms of writing legislation and moving legislation."
Holt has pushed back against increased surveillance - proposing to repeal the Patriot Act - and has spoken out against the use of drones in combat.
"Wouldn't it be good to have at least one scientist" in the Senate, he asked, pointing to Obama's speech on climate change Tuesday. "I don't have any trouble laying out a distinctive record of accomplishment."
Each said his grassroots support, and potential to drive turnout, could make the difference when up against Booker's statewide popularity.
As they battle over the left, Booker plans to launch a television ad Wednesday promoting his ability to "bring people together."
"Washington ducks our problems," he says to the camera. "I won't."
Oliver, a black woman from East Orange, may reach a different set of voters from Pallone or Holt.
"I'm the only woman who is in this race, and I think that immediately distinguishes me from the other candidates," Oliver said. As Assembly speaker for the last four years, she added, she is the only one who has had a statewide portfolio.
But she is from the same county as Booker, doesn't have the fund-raising prowess of the two congressmen, and is off to a late start after spending recent weeks focused on state legislation, including the budget approved this week. She plans to fully devote herself to the race after this week.
"The short time frame certainly puts me at the broadest and greatest disadvantage," Oliver said. "I had not anticipated getting into an election cycle in such a short period of time."
There are just seven weeks until the primary. Unless one of the Democrats emerges as a clear alternative to Booker, the race could turn into a rout.
Contact Jonathan Tamari at firstname.lastname@example.org. Follow on Twitter @JonathanTamari. Read his blog 'Capitol Inq' at www.inquirer.com/CapitolInq.