Earl Krugel, 60, a dental hygienist, will make the plea in exchange for a 10- to 20-year term rather than a mandatory 40-year sentence.
His co-defendent, Irv Rubin, the national chairman of the JDL, died last November after cutting his throat and leaping from a balcony while awaiting trial in jail in Los Angeles.
Krugel and Rubin were arrested in December 2001 after an FBI informer had delivered explosives to Krugel and had taped discussions of the plot. Bomb parts, instruction manuals and anti-Arab literature were also found at Krugel's house.
Rubin, chairman of the JDL since 1985, had allegedly given the informant, a young JDL recruit, instructions to bomb the King Fahd mosque in Culver City, south of Beverly Hills.
A list of other "filthy mosques" - as Krugel called them on tape - had been drawn up to give Arabs what he described as a "wake-up call".
Part of the plot was to blow up the San Clemente offices of Darrell Issa, a Lebanese-American Republican congressman. There was also a plan to blow up the Muslim Public Affairs Council in Los Angeles.
Mr Issa welcomed the plea. "Americans reject extremists who seek influence through the use of terror, whatever the source, at home and abroad," he said.
The King Fahd mosque, where security has been stepped up since the plot became public, also welcomed the plea.
Krugel, who has recently married a French woman, will agree not to have contact with any JDL members or former members after his release as part of the deal. His lawyers said he had accepted the deal because the climate in the US was not sympathetic to a defendant in a terrorism case.
Founded in 1968 by Rabbi Meir Kahane, the JDL once had 18,000 members with its logo of a fist inside the Star of David and its slogans of: "Every Jew with a .22" and "Keep Jews alive with a .45."
But membership has dwindled as the league has been shunned as an embarrassment for its extremist views by mainstream Jewish organistions in the US.
Part of the reason for the bomb plot, according to the FBI informant, a 22-year-old former member of the US Navy called Danny Gillis, was Rubin's desire to show that the JDL was still "alive in a militant way."
