When it comes to barbecues, farm-town-girl-turned-perennial-Democratic-fund-raiser Esther Coopersmith believes in giving them for everyone: The Lyndon Johnson, the Egyptians, the foreign press, even Shakespearean scholars.

Yesterday it was the Israelis' turn at yet another barbecue, this one given for Ephraim Evron, the Israeli ambassador to Washington.The heat? "Well, my dear," said Coopersmith, "it could have rained."

Besides, most of the 100 or so guests huddled comfortably inside the Coopersmiths' air conditioned Potomac, Md., home. The only one who really suffered was Jonathan Coopersmith, 23, forced by his mother to greet the arriving ears.

"This is what a degree at Princeton gets you," he complained. "The chance to stand down at the bottom of a driveway on a hot day directing traffic."

Inside where it was cooler, guests greeted the new ambassador, dressed casually and tieless like everybody else. (Actually, the "new" ambassador has been here since January, but this is the first opportunity the Coopersmiths have had to welcome him officially. Life has been hectic lately, Esther Coopersmith explained.)

Greetings done, the talk turned to SALT, the weather, squash and, naturally, speculation on what Jimmy Carter would say on television several hours later.

Sen. J. Bennett Johnston (D-La.), who co-hosted the party, claimed he didn't know, nor did Sens. Walter Huddleston (D-Ky.), Alan Simpson (R-Wyo.), Dennis DeConcini (D-Ariz.), Gaylord Nelson (D-Wis.) and Rep. Al Ullman (D-Ore).

Ullman, however, predicted the speech would be dull, dull, dull. "It's going to be a letdown because Jimmy Carter is Jimmy Carter," he said. "On the other hand, I'm hopeful that the president will give us something we can rally behind."

Dr. Melvin Gottlieb, director of the Plasma Physics Laboratory at Princeton University, was hopeful, too. Asked about Carter's speech, he stopped momentarily from his cocktail rounds and launched into a tirade on the future of energy.

"I just can't wait to hear his speech tonight," he said, clenching his fist. "Damn it, hit it hard."

John White, chairman of the Democratic National Committee, confessed that he also didn't know the specifics of Carter's speech because he was eating and drinking at a barbecue instead of talking to Jody Powell. "I couldn't resist Esther's barbecue," he said.

Energy Secretary James Schlesinger showed up at the party after almost everyone else had left.Before sitting down to his barbecued chicken, he told a small circle of listerners that Carter's energy program would be a "massive" one.

"I think that the country will react to the challenge - somewhere between Paul Revere's ride and Pearl Harbor."

Asked whether he's thinking of quitting his job, a favorite rumor in Washington these days, Schlesinger gave his now stock reply: "I'll be around as long as I'm useful." But does he like his job? "It is a national obligation".

By 6 p.m., most guests had wandered out to poolside to eat 110 pounds of chicken, 75 pounds of beef, and 132 ears of corn. Only four guests played tennis beforehand, and only three, including Sen. Larry Pressler (R-S.D.) had gone swimming. "I swam five laps," said Pressler proudly. "Maybe five," corrected Pressler's friend Diane Tracy.

As for the Israeli ambassador, the very much American barbecue was hardly a novel event for him. Evron, you see, once spent a weekend at the LBJ ranch in Texas. "Very interesting and very tasty," he said. CAPTION: Picture, Sen. J. Bennett Johnston and Esther Coopersmith, rear; Mrs. Evron, Mrs. Johnston and Ambassador Evron, front left to right. Photo by Gerald Martineau