INDIANAPOLIS, Ind., May 5--The Democratic primary campaign in the Hoosier State is fast becoming a stop-Kennedy effort.

With only two days remaining before this May 7 primary, the big question is the size of Robert F. Kennedy's vote. One source high in the New York Senator's staff confided Saturday that their latest poll shows Kennedy reaching over the 50 per cent mark for the first time.

Many feel that this might seriously injure Senator Eugene J. McCarthy's campaign in the other primary contests with Senator Kennedy.

One high McCarthy source, who preferred to remain anonymous, said before leaving Indiana late this afternoon, "I don't want to be here Tuesday evening for the debacle." Other key Indiana McCarthy staffers are leaving the state by train Tuesday morning.

While showing gains in black middle-class neighborhoods and some other areas, the McCarthy canvassing results from this past weekend's 7000-man effort have not been encouraging. Michael Devorkin, a Kennedy student volunteer leader, said, "Our canvassing has been really encouraging this weekend. It looks good for the election."

Hoosier Governor Roger D. Branigin's provincial "Indiana-for-Hoosiers" campaign seems also to be faltering; the Governor cancelled all remaining campaign dates this afternoon without explanation. Many of Branigin's powerful Democratic machine workers are defecting to the Kennedy camp.

In an apparent effort to cut Kennedy's expected plurality, McCarthy lashed into the New York Senator for the first time Friday evening in an suburban Indianapolis speech. He attacked Kennedy for misrepresenting his Congressional voting and attendance records, called on him to reveal his controversial Indiana primary expenditures, and challenged him to debate late Saturday afternoon on Main Street in Scottsbluff, Neb.

But Kennedy didn't appear for the showdown on Main Street; instead the New York senator spent his afternoon speaking before large, wildly enthusiastic crowds in white and black areas of Indianapolis