Why should that be "indefensible"? One might consider Julius Rosenberg's act a small service to humanity, for which he _and his wife_ were made out to be enemies of the people and murdered by the state. While I believe Soviet scientists would have developed atomic bombs on their own anyhow, why should trying to help them to expedite the process be considered "indefensible" by _leftists_?
I just showed _The Atomic Cafe_ to my students. You can see, in this documentary, American politicians arguing for the first strike. Without the speedy Soviet development of atomic bombs, the USA might have dropped them on more places than just Hiroshima and Nagasaki.
BTW, that America used the atomic bombs to show them to the Soviets, not to compel the Japanese to surrender (which they were going to do soon anyhow), is still verboten in official discourse, as the case of the censorship of the Smithonian exhibition attests. American leftists might work harder to stop the U.S. government and its stooges from defending the indefensible. Further, American leftists should focus on destroying the National Security State here and now -- the enemy of humanity _and_ American liberties.
Yoshie