The head of El Salvador's leading right-wing political party was slightly wounded today by unknown gunmen in the second attack on his group in 24 hours. Former National Guard major Roberto D'Aubuisson, 38, head of the Arena party, was grazed on the right shoulder by a single shot from a light truck that passed his small motorcade as it left the capital's Ilopango Airport, according to a party spokesman.

Attacks on d'Aubuisson are likely to reinforce the message he has been pounding home at his well-attended rallies: that his party is the only one that leftist guerrillas really fear.

A party spokesman blamed the attack and another one at party headquarters last night on the Christian Democratic Party of President Jose Napoleon Duarte. A Christian Democratic spokesman rejected the accusation as "totally irresponsible." D'Aubuisson repeatedly has accused the Christian Democrats of being "red on the inside."

D'Aubuisson tells rallies that if Arena wins the March 28 elections for seats in a constituent assembly, it will name him president and he will "clean up the country" in three months.

Christian Democrat leader Julio Adolfo Rey Prendes has called D'Aubuisson's party "fascistic." Christian Democratic officials say privately that if Arena wins, 3,000 to 5,000 Christian Democrats would flee the country in fear for their lives.

In the attack last night, five party officials were wounded as they left party headquarters when a passing pickup truck opened fire and then tossed a grenade, Arena officials reported. The wounds were serious, but the five will live, hospital officials said.

Doctors who treated D'Aubuisson at the private San Salvador Clinic said he had suffered a flesh wound, was bandaged and released, walking to his car. "It was nothing serious," one said.

D'Aubuisson's car and two other vehicles of party officials had been on their way to a party rally in San Vicente, northeast of the capital, when they stopped for gas, Arena spokesman Mario Redaelli said. D'Aubuisson got out of the car and was standing by it when the truck came by and men in it opened fire, apparently with a pistol, Redaelli said.

D'Aubuisson's group had just left Ilopango Airport, where they had lunched after trying unsuccessfully to helicopter to an earlier rally at San Sebastian, north of the capital. A battle, presumably between guerrillas and armed forces units, was going on just outside the city and the helicopter turned back, Redaelli said.