Scathing words between Mayor John DeStefano Jr. and Jeffrey Kerekes, his challenger in November’s mayoral election, made Wednesday the most heated day of the campaign so far.

A tax dispute between the city and Bruce Becker SOM ’85 ARC ’85, the architect and developer of 360 State St., served as a catalyst for an exchange of barbs between the candidates that quickly turned personal. By taking Becker’s side in a lawsuit that alleges the city improperly raised the tax assessment on the landmark downtown building, Kerekes showed himself to be “in the pocket” of downtown developers, DeStefano said at a campaign rally. Kerekes hit back even harder in an interview Wednesday night, accusing the mayor of imperiling the city’s long-term economic development by “changing the rules” after luring Becker to New Haven.

A report by the city’s tax assessor released this week estimates that 360 State St., which contains 500 apartment units, will eventually have to pay $5.7 million annually in taxes — four times the amount the city projected when trying to convince Becker to build it. Because of the lawsuit, that revenue is in question, and the outcome of the suit will determine the validity of one of DeStefano’s main campaign talking points: that he has presided over record growth in the city’s tax base in an economic recession. While New Haven netted $149 million in grand list growth last year, if courts agree that the city broke the law in assessing the building’s tax bill, then over two-thirds of that growth could be wiped out.

The dispute puts the future of the city’s economic development in jeopardy, Kerekes said.

“This extends far beyond Bruce Becker,” he said. “Who’s going to come and invest in our city when you feel like there’s a war on developers? If you’re a developer, and you get slammed the day you finish your project, that hurts our development.”

DeStefano denied that the assessor acted improperly, countering that Kerekes is showing himself to be an “apologist for big developers.” The suit amounts to a demand for a corporate tax break, he said, suggesting that Kerekes’ support for it stems from his desire for political favors from Becker and other developers.

“I guess [Kerekes] wants support from anyone he can get it from now,” he said.

The contest between 18-year incumbent Destefano and Kerekes, a budget watchdog from Wooster Square, is set to be the most bitterly contested election the city has seen since 2001, when the mayor defeated State Senate Majority Leader Martin Looney in a primary. The mayor’s heated words Wednesday afternoon reveal a growing fear for his political life, Kerekes said.

“He’s feeling very afraid, and he should,” Kerekes said.

The occasion for DeStefano’s zingers was a rally at the East Rock School on Willow Street, where members of the Connecticut Laborers’ Local 455 congregated to support the mayor’s bid for a record 10th term in office. The rally, which drew about 80 uniformed construction workers despite heavy rain, served as a reminder that while anti-DeStefano rhetoric was pivotal in sweeping a slate of union-backed to aldermanic primary victories, the mayor’s ties to organized labor run deep. Local 455 is just one of many union endorsements DeStefano has wrapped up in recent weeks.

While DeStefano cruised to victory over Kerekes in the Democratic primary Sept. 13, his failure to obtain more than 43 percent of the vote suggested his reelection is in more doubt than it has been in a decade.

The mayor’s reelection bid has relied heavily on the contributions of city employees and contractors, Kerekes said. An examination of his campaign’s latest finance report confirms DeStefano has received $20,600 from individuals with city contracts and $5,350 from city employees. In total, those contributions represent only 6 percent of DeStefano’s campaign coffers.

In part because of his decision to opt out of the city’s public financing program, DeStefano has been able to raise $425,937, nearly 13 times the money at Kerekes’ disposal.

While Kerekes lost in the Democratic primary with less than a quarter of the vote, he will compete in the Nov. 8 general election as an independent.