WASHINGTON--Sen. Paul E. Tsongas (D-Mass.) said yesterday he won't stand in for Sen. Edward M. Kennedy '54 (D-Mass.) because he "cannot conceive" of Kennedy's not challenging President Carter.

In the past Tsongas has portrayed himself as a "stalking horse" for Kennedy, a role in which he would run for Kennedy so as to hold the state's delegates' votes in reserve if Kennedy enters the presidential field late, or in the case of a Kennedy draft at the Democratic national convention.

Kentucky Derby

However, he said recently, "Why be a stalking horse when there's a thoroughbred available?"

Tsongas added that his candidacy in the presidential primary as a stand-in for Kennedy would be "unnecessary and indeed irrevelant" in light of Kennedy's recent statements concerning the 1980 presidential election.

Kennedy has until January 4 to file nomination papers for the Massachusetts primary.