Even Republicans had expected Booker, a Democrat in a Democratic-leaning state, to cruise to victory by a wide margin over little-known Republican Steve Lonegan in the special election to replace former Sen. Frank Lautenberg, who died in June.
While Booker holds a double-digit lead in most polls, the charismatic Newark mayor has faced sustained Republican criticism that has exposed vulnerabilities that could hamper him should he seek even higher office someday.
Lonegan has hammered Booker on Newark's economic troubles, tax increases, and violent crime. The GOP also has assailed him over a 2008 statement that a drug dealer he called a friend was actually an "archetype"; his G-rated Twitter exchanges with a Portland, Ore., stripper; his out-of-state fundraising trips; and a Washington Post interview where Booker, who talks about past girlfriends but prefers to keep his personal life private, said he "loves" when people on Twitter say that he is gay and asked, "so what does it matter if I am?"
After weeks of mostly ignoring Lonegan, the sustained assault has gotten Booker's attention.
He has aggressively hit back in the past several weeks, castigating Lonegan seemingly at every turn, using the brass-knuckled political skills he learned in the rough-and-tumble world of Newark politics.
"Sending him to Washington would be like pouring gasoline on a fire," Booker said, calling Lonegan a member of the "tea party fringe" that "hijacked" the government and caused a shutdown.
In a debate last week, Booker said the former mayor of Bogota, a small borough in Bergen County, "ran his city into a ditch" and asked for a state bailout. He painted Lonegan as an extremist, and said sending another Republican who supports the government shutdown to Washington would hurt the country.
Before deciding to return heavy fire, Booker had focused more on policy differences with Lonegan on issues like child poverty and criminal justice reforms, painting himself as a political uniter, while promoting Newark's growth. He told Lonegan to "bring on your wrecking ball" after the primary, but almost immediately shifted toward highlighting policy differences, fundraising and disregarding attacks. Booker largely avoided local media interviews, but held regular "run with Cory" events, where a group of supporters jogged a mile with the mayor.
For most of the campaign, Lonegan has gotten little help from Republicans outside the state, save for endorsements from Kentucky Sen. Rand Paul and Texas Gov. Rick Perry, both tea party favorites. The Senate Republican campaign arm hasn't sent any staffers into New Jersey - a standard practice in competitive races - and hasn't spent money on radio or television ads.
Booker had raised $11.2 million for his campaign through early October, compared to Lonegan's $1.4 million, according to campaign finance reports reviewed by the Newark Star-Ledger.
In an 11th hour push for Lonegan, tea party leaders have begun coordinating phone banks and a get-out-the-vote effort. The nation's largest tea party political action committee - the Tea Party Express - brought former Alaska Gov. Sarah Palin in to campaign for the GOP nominee last weekend.
Tea party supporters dream of another surprise upset like Republican Scott Brown's unexpected victory in Democratic-leaning Massachusetts in a 2010 special election to replace the late Sen. Ted Kennedy.
Democrats in Washington mostly stayed out of the race until the final week. President Barack Obama released a video Monday urging voters to cast ballots for Booker and Democratic National Committee Chairwoman Debbie Wasserman Schultz stumped with Booker on Sunday. New York Mayor Michael Bloomberg, a longtime Booker supporter, spent $1 million on a television ad for Booker.
Booker campaign officials say they expected all along that the race would tighten, because no New Jersey Democrat running statewide has won with more than 60 percent of the vote in a generation. But they say they're confident that the double digit lead in the polls will be borne out on Election Day.
"We knew there would be a narrowing," Booker said in an interview with The Associated Press, "and so far the election has gone for us exactly according to plan."
Still, Republicans in Washington say they're pleased that Booker has had to work harder than anyone imagined. They're privately cheering the tea party's involvement.
And they suggest that Booker is making mistakes that could come back to haunt him as he eyes his political future. Some Democrats have mused about the possibility that Booker - a gifted public speaker who is young at age 44 - could make an attractive vice presidential candidate in 2016.
There's little doubt that Booker has national aspirations. He's spent a chunk of his mayoral tenure traveling the country, meeting with big Democratic donors and raising money in places like Hollywood and Silicon Valley. Hollywood star Matt Damon helped organize one Booker fundraiser in California.
Booker advisers suggest that the aggressive fundraising schedule has dual benefits, generating resources quickly for the special election, while giving Booker a head start for his next election. If he wins on Wednesday, he'll have to defend his seat next November.
Lonegan, however, says his campaign is "cresting."
"It's not a longshot," he told the AP. "We're going to win on Wednesday."