The American Legion yesterday joined the Veterans of Foreign Wars in endorsing a House bill that would compensate Vietnam veterans for exposure to Agent Orange if they suffer from one of three diseases.

The bill, sponsored by 113 House members, was introduced yesterday by Rep. Thomas A. Daschle (D-S.D.), who said the government would be irresponsible if it bought the homes and businesses of the 2,400 people of dioxin-contaminated Times Beach, Mo., but refused to compensate the nation's 2.8 million Vietnam veterans. Agent Orange, a defoliant, contained the same toxic chemical at levels 20 times greater than found at Times Beach, Daschle said.

Similar compensation bills have been introduced during the past decade, but this is the first to be endorsed by the nation's two largest veterans' groups, which have a combined membership of 4.5 million.

The action caused concern yesterday among key staff members of the congressional veterans' affairs committees. Both committees have refused to support compensation bills because they say they believe there is no conclusive scientific proof that exposure to Agent Orange causes health problems.

"It isn't going to be easy for either committee to hold the line now," a senior member of the Senate Veterans' Affairs Committee said yesterday.

National Commander Al Keller Jr. said the Legion will endorse Daschle's bill because "scientific evidence is available pinpointing three medical disorders that may be caused or aggravated by exposure to the herbicide."

That surprised top Veterans Administration officials because veterans' groups have known about the "scientific evidence" since 1981.

Under the bill, the Veterans Administration would "presume" that three disorders were caused by exposure to Agent Orange: cancer of some soft-tissue organs such as lung, stomach and muscles; porphyria cutanea tarda, a condition that affects the liver, blood and skin; and chloracne, a severe skin disorder.

Veterans would be eligible for compensation if they suffered from one of those disorders and if they could prove they served in areas sprayed with herbicides during the war.

Daschle estimated that 2,500 veterans would be eligible for compensation and that it would cost $3 million to $4 million per year. Their benefits would be terminated if government studies now under way eventually show no correlation between Agent Orange and the ailments.

The VA has granted "presumptive disabilities" in connection with 40 diseases presumed to be service-related. It has received 16,564 disability claims from Vietnam veterans who blame Agent Orange exposure for health problems. It has granted 1,275 claims, but insists that none of the health problems was caused by Agent Orange.