Threatened by possible Republican Party jamming of the telephone lines and a host of last-minute squabbles, organizers of the Democratic Party's Memorial Day weekend telethon go into the $6 million extravaganza Saturday night hoping at least to break even by the time the 17-hour, nationwide broadcast is through.

Some on the organizing committee for the event, entitled "Celebrate America," said a trouble-shooting squad from the Democratic National Committee in Washington had to be sent in to work out taping schedules and find enough stars to draw viewers to what otherwise would be history's longest political commercial.

Until just a few days ago, the small number of celebrities signed up "was just embarrassing," said one DNC staff member. Former California governor Edmund G. (Jerry) Brown Jr. had to be enlisted to round up actors Edward Asner, Jane Fonda and Candice Bergen.

Other organizers, particularly those close to Kentucky Gov. John Y. Brown Jr., chairman of the organizing committee, said the telethon staff had needed no help. They said Brown was simply being harassed by DNC officials who disagreed with his efforts to make the show entertaining and nonpolitical.

Democratic Party officials said they could not predict the impact of a Republican appeal to supporters of President Reagan to jam the telethon phone lines with complaints about anti-Reagan statements. Telethon organizers said they were not carrying a heavily partisan message, due to network restrictions, but Sam Goddard, chairman of the Arizona Democratic Party, said he was trying to raise money before the telethon in case the Republican jamming caused it to be a fund-raising bust.

The Democrats, who have raised about $7.3 million with four previous national telethons, expect at least enough telephoned pledges during the NBC broadcast to recoup the network's $3 million airtime charge and another $3 million spent on production. Telethon spokesman Josh Baran said party officials anticipate that any funds for 1984 Democratic campaigns will come from showings of a three-hour version of the telethon in local areas over the next year.

Brown said the party placed even more importance on getting the names and addresses of those who pledge money this weekend so they can be contacted in the future. Scott Nelson, western regional director for the DNC, said the party now has 250,000 names on its mailing list, "and doubling that would be a success to us."

The telethon will be shown on Channel 4 in the Washington area beginning at 9:30 p.m. Saturday. Like other local stations carrying the full 17 hours, Channel 4 will show its regular 11 p.m. news and then return to the telethon until it ends at 3 p.m. Sunday.

The telethon will feature live performances or appearances by singers Helen Reddy, Waylon Jennings, Kris Kristofferson and Sergio Mendez; actors Ted Danson, Anthony Geary, Sally Kellerman and Asner, and comedians Pat Paulsen and Norm Crosby.

Actors Mary Tyler Moore, John Forsythe, Paul Newman, Dennis Weaver, Bea Arthur, Ben Vereen and Fonda; singers Willie Nelson, Dottie West and Carmen McCrae; the cast of the musical "Dreamgirls," and magician Harry Blackstone will appear on tape. Organizers rejected a plan for two live rock concerts proposed by Democratic pollster Pat Caddell.

Leslie Uggams and Daniel J. Travanti will host the broadcast, which will also show a biography of the late President John F. Kennedy and a series of "mini-documentaries" on current American problems. About 2,000 telephone operators in about 18 phone centers around the country will take pledges from viewers who can call toll-free numbers flashed on the screen. Unlike traditional telethons, Baran said, the broadcast will not show a running total of pledges or have telephone operators on camera.

Republican National Chairman Frank Fahrenkopf Jr. last week mailed Republicans a warning of a "savage and self-serving Democrat attack" by "Hollywood left wingers" and suggested jamming the lines "to let the Democrats know you support President Reagan."

Democratic Party Chairman Charles T. Manatt complained bitterly about the mailing but telethon organizers rejoiced at the free publicity.