The election of former army major Roberto d'Aubuisson as president of the Constituent Assembly of El Salvador was the worst possible outcome. But the administration is learning to love the charismatic killer in the elevator boots and plans to have him come here in the near future. D'Aubuisson managed to get himself thrown out of the Salvadoran Army for human rights violations, which is not an easy thing to do, but raptures about the election have overcome doubts about his character.

While liberal Democrats insist they are not bluffing about a cutoff in U.S. aid if d'Aubuisson tries to keep his campaign promise to "exterminate" the guerrillas in 90 days, the little major is hearing a different message from U.S. Ambassador Deane L. Hinton.

U.S. aid, Hinton said Friday, "will continue in any case."

It is doubtful that d'Aubuisson, unquestionably the political rookie of the year, will change his views or his tactics. He came out of nowhere six months ago to found the Nationalist Republican Alliance party, known as ARENA, and to gain 29 per cent of the vote in El Salvador's experiment in democracy, as the calamitous election of March 28 was called until the ballots were counted.

Efforts are now being made to change his image. The firm of McCann-Erickson, which handled his campaign, is probably engaged for the public relations aspect of d'Aubuisson's forthcoming visit here. An Immigration and Naturalization Service entry ban, instituted against him after he flew here in 1980 although his visa had been revoked, will be lifted for the occasion. During his visit, he will be heavily chaperoned and instructed to speak only from official texts.

Surely, he will be cautioned to modify his enthusiasm for Adolf Hitler. According to the Mexican paper El Dia, d'Aubuisson told three European reporters, "You Germans are very intelligent. You realized that the Jews were responsible for the spread of communism, and you began to kill them."

During the campaign, d'Aubuisson said the only thing he wanted from the United States was napalm to wipe out the left.

No one in Washington will be tactless enough to ask him about his alleged part in the assassination of Archbishop Oscar Romero. A year ago, our former ambassador to El Salvador, Robert C. White, gave to a congressional committee evidence he called "compelling if not 100 per cent conclusive" that d'Aubuisson ordered the killing.

The State Department pursued the case with all of the vigor it showed in solving the murder of four U.S. missionaries more than a year ago.

D'Aubuisson was the founding father of the White Warriors Union, a death squad that plotted a coup against our former favorite Salvadoran politician, Jose Napoleon Duarte. D'Aubuisson calls Duarte a communist.

But with the coaching of Hinton, d'Aubuisson is learning to talk nicely and discovering how easy it is to take into camp Americans who want to believe.

House Majority Leader James C. Wright Jr. (D-Tex.) came away with a favorable impression of d'Aubuisson after sitting next to him at a recent post-election dinner at the U.S. Embassy in San Salvador.

"He is an intense, keyed-up young man," Wright said, "but I believed him when he said he wants a government of national reconciliation. He says he will offer amnesty to the guerrillas and would do so in all cases except those involving the crimes of murder and kidnaping. In those, he said he could not forgo punishment."

Wright found the 80 percent turnout in the Salvadoran election "an inspiring damn thing" and agrees with the administration interpretation of the results as a "repudiation of the left." Since the left was not represented on the ballot, that particular good news was guaranteed in advance.

What the outcome also guaranteed is continuation of the civil war. As Michael D. Barnes (D-Md.), the scholarly sophomore who is chairman of the House Inter-American Affairs subcommittee, told an audience at the Women's National Democratic Club, "The left is still there and still militarily strong."

Barnes added somberly that Christian-Democrats who have known d'Aubuisson longer "may feel they have no choice but to join the armed guerrillas."

"By insisting on a hasty and deficient election," White says, "we have given a blank check to the crowd in El Salvador that wants a military victory."

Chances for negotiations have vanished. Sens. Paul E. Tsongas (D-Mass.) and Christopher J. Dodd (D-Conn.), rounded up only 14 cosponsors for a bill that would mandate negotiations as a condition for further U.S. military and financial aid.

Thomas O. Enders, assistant secretary of state for Inter-American affairs, said the bill would "have a negative impact" on the delicate process of forming a coalition government in El Salvador.

Once again, we are making a buddy out of a thug. This one may be worse because we have, by insisting on elections, made him legitimate. And already the Reagan administration, beguiled by his anti-communist fanaticism, has shown how little influence it will have on him. It probably will have about as much as Reagan had with Argentine President Leopoldo Galtieri, another violent man of the right.