There are signs of panic among members of the Democratic establishment, who worry Mr. Barron could prove to be a headache in their ranks and an alienating figure on the national stage.

Popular Democrats abruptly emerged this week to denounce him as a dangerous, anti-Israel radical. Edward I. Koch, the former mayor, called him a viper; other community leaders pointed reporters to the Anti-Defamation League’s list of his more provocative quotes; and in an e-mail to supporters this week, a local group of Russian Jews announced a hastily planned rally on Monday to denounce Mr. Barron as “a fringe radical and anti-Semitic, anti-Israel activist.” The first word of the subject line said it all: EMERGENCY…

In some ways, the race offers a contrast between two different eras. Mr. Barron, 61, represents a throwback to the 1970s and 1980s, when black nationalists seemed to control the city’s racial conversation, while Mr. Jeffries, 41, represents the more recent model of black leaders like President Obama; Newark’s mayor, Cory A. Booker; and Gov. Deval Patrick of Massachusetts, who have earned establishment credentials and thrived by building coalitions with white liberals.

That difference was suggested by Congresswoman Yvette D. Clarke as she campaigned with Mr. Jeffries at the Abe Stark Senior Center in Canarsie. She said that Mr. Barron had a “style of politics that appeals to some groups” but that Mr. Jeffries represented “the future of Brooklyn.”