Some Democrats believe his campaign has at least temporarily damaged Booker's well-honed image and sense of limitless potential, even as nearly every pundit and operative expects Booker to win a double-digit victory in Wednesday's special election.
"The bloom is off the rose, for sure," said State Sen. Ray Lesniak (D., Union). "But he will have plenty of time to rebound as our next United States senator."
Booker is still heavily favored to win, and Lonegan has threatened to falter in recent days, but five Democratic operatives and officials criticized the Newark mayor's overall performance in the campaign. Several said Booker's showing could embolden challengers in 2014, when this year's winner will have to run for a full term.
The criticisms of his campaign have mirrored complaints that dogged Booker as mayor: that he would rather enjoy the celebrity limelight than do gritty day-to-day work, and that after years of glowing media coverage, his mythic image couldn't stand up to the scrutiny of a statewide race and a relentless opponent.
The critical Democrats, who did not want to be named blasting their party's Senate nominee, saw a candidate who expected to walk to victory.
"Lonegan successfully put the focus on Cory and exposed some of the warts that everybody has," Lesniak said. "No one could be as good as Cory was perceived to be, no one."
Two allies said Booker was slow to go on the offensive because he was focused on protecting his "brand" of positivity.
The night of his primary victory, Booker vowed to match Lonegan's acerbic attacks "with positive vision." He also may have been trying to starve Lonegan of attention.
But the approach took a toll.
Independent polls last week showed Booker with an 11- to 12-point lead - and trailing Lonegan among independents, according to a Quinnipiac University survey. Overall, it's a comfortable margin, but smaller than many expected against a fire-breathing conservative in a state President Obama won last year by 17 points.
The concern over Booker was illustrated by New York Mayor Michael Bloomberg's recent $1 million ad buy to help his ally, even though Booker had spent nearly $9 million in the primary and general elections by the end of September, compared with $1.1 million for Lonegan.
Jolted into action, Booker has become newly aggressive in the campaign's final days, painting Lonegan as an "extremist."
And with Lonegan seemingly feeding his opponents' strategy by filling his public appearances with conservative red meat, even Booker's critics say the mayor could finish strong.
Lonegan's provocative debate performance Wednesday, referring at one point to bodies floating in the river around Newark, followed by a rant later in the week by his close adviser Rick Shaftan - who suggested Booker was gay and was quickly fired - gave the impression of an upstart campaign fumbling at the finish.
"Anyone running for the United States Senate would be happy to have a double-digit lead a week before the election," said a Booker campaign memo written last week.
While not a direct comparison, New Jersey's other political star has cast a shadow over Booker's results. Gov. Christie leads his reelection race by roughly 30 points.
Booker "didn't really engage in the campaign in a way you need to engage in a New Jersey campaign," said Patrick Murray, director of the Monmouth University Polling Institute.
Montclair State political scientist Brigid Harrison called Booker's campaign "lackadaisical."
"I'm not quite sure what they were doing," she said.
When Booker did make headlines, it was often for the wrong reasons: He sent Twitter messages to a Portland, Ore., stripper, gave a reporter a rambling answer about his sexuality, and faced questions about the truth behind some of his heroic stories in Newark.
Some Democrats, however, said Booker's superstar status still resonates. They say that the mayor made a strategic decision to focus on fund-raising and that the race's early start - brought on by the death of Sen. Frank Lautenberg in June - forced him to the fund-raising circuit during prime campaign time.
The widespread criticism of Booker's run is based on "structurally unrealistic" expectations given the short race and an unusual mid-October election day, said the campaign memo, written by strategists Brad Lawrence, Steve DeMicco, and Danny Franklin.
Significant strengths remain: In Quinnipiac's latest poll, Booker's favorability rating was a robust 57 percent, and his big financial advantage, built in part while he was away, could be crucial as he shines a spotlight on Lonegan's views in the campaign's stretch run.
"I was very happy with how Lonegan represented himself because tonight he told his truth for everybody to see," Booker said after the debate.
That night, Lonegan proudly staked out right-wing positions on same-sex marriage, abortion, and federal spending - vowing to gut the Department of Education, among others. He cheered Sen. Ted Cruz (R., Texas) and the ongoing government shutdown.
But his positions are unlikely to appeal to moderates, whom Republicans need in a state where Democrats hold a 732,000-person edge in voter registration.
Lonegan's unfavorable ratings grew 11 points in two weeks between late September and early October, according to a Quinnipiac poll out Wednesday.
Booker's admirers predicted that he would quickly put the turbulent campaign behind him.
With Booker's soaring oratory, allies predicted that he would quickly settle into the Senate and restore any lost luster.
But first he has to prove he can finish off a race that has tested his political strength.