The plain black car pulled to the entrance of the Landover office building just after 8 on Wednesday night. The tall man leapt out. The famous voice issued forth:

"You be back here in 10 minutes! Ten minutes!," said the voice, which belonged to Sen. Edward M. Kennedy (D-Mass), who was running late. "And make sure you figure out how to get us out of here."

Kennedy, or "The SEN-ator," as he was reverentially called throughout the evening, dropped into the $50-per-plate fund-raiser Wednesday night to boost the candidacy of Thomas R. Hendershot, a New Carrollton lawyer who kicked off his campaign for a seat on the Prince George's County Council. Hendershot, 38, ran Kennedy's 1980 presidential race in Prince George's and helped him win the county by 4,000 votes. He also knew the senator from Hendershot's Capitol Hill days as an aide to former congressman William Green, now the mayor of Philadephia.

Hendershot pointed out that Kennedy had visited the county twice before, but never before for a council race. How did Hendershot manage that? "I asked him," shrugged Hendershot. "All he could do is say no."

Kennedy was only the latest to hop aboard the celebrity bandwagon rolling through Prince George's in recent weeks. Last Friday, President Reagan hosted a luncheon for 75 sympathetic black ministers to defend his administration against charges of racial insensitivity. The party included no less than a dozen Prince George's preachers who recently converted to the GOP.

Two weeks ago, Republican National Chairman Richard Richards feted the same county ministers at the Capitol Hill Club before television cameras and microphones, proclaiming that their presence was "one of the most significant things that has happened to the Republican Party in many, many years."

One of the ministers, the Rev. Perry Smith of the First Baptist Church of North Brentwood, is an undeclared candidate for Congress in the 5th Congressional District, where he would face incumbent Democrat Steny Hoyer.

Not to be outdone, Smith's primary opponent, James Whitehead, Jr. who is an aide to supply-side wizard Rep. Jack Kemp (R-N.Y.), decided to hold a little party of his own. He invited soul singer/composer Isaac Hayes to fly up from Atlanta to entertain a $35-per-plate crowd at the Sheraton New Carrollton this Saturday night.

"It just worked out with the luck of God that Isaac Hayes is going to do all this for me," gushed Whitehead, who also is having popular WHUR radio host Jerry Phillips to serve as master of ceremonies.

Whitehead said that Hayes is not a Republican and would receive only expenses. "He's supporting me more for my personal principles than because of my party," said Whitehead, whose principles include a profound belief in free enterprise and conservatism.

The mania for celebrities, according to a prominent zoning lawyer at the Kennedy-Hendershot function, demonstrates how far candidates must be willing to go in this slateless, wide-open election year in Prince George's. Already complaining about the large number of hands outstretched for money, he likened the council race to "a duck shoot at nine targets, and we're supplying the pellets."

Kennedy, preparing the crowd for Hendershot with some patented Democratic whistlestop patter, reflected that view.

"Whenever we've talked about the economy, we talk about demand side and supply side. Well, tonight Tom is on the demand side and you," he said to laughter and applause, "are on the supply side."