As usual, it depends on your POD.
FDR was very pro-Soviet and his administration was filled with other pro-Soviet individuals - no few of which were in the service of or were being used by the Soviet Union itself.
For any such anti-Communist and anti-Fascist stance to be practicable, it'd have to either not be FDR at all or to have a significantly changed FDR to start with. That, in turn, would've meant for some vast policy differences at the Federal level during the 30s. So, this ATL would be - out of necessity - much more broadly different than just having an anti-Communist FDR being the only thing different.
Perhaps still have the same FDR winning in '32 and starting off as in OTL. But then he fully learns of the Holodomor and then learns of the Soviet's "Gulag" death camps. This causes him to profoundly reshape his view of the Soviet Union and of Communism in general. Couple this with Hoover's - as in J. Edgar - informing FDR of the massive Soviet penetration of his administration and of the Federal government in general, and this leads to some substantial changes. Changes which soon take on a life of their own as each individual anti-Communist action and policy incites further anti-Communist actions and policies.
Thus the Dies Committee gets formed - but then Dies gets revealed as having those Soviet connections and is forced to resign. His replacement is thus compelled to be zealous in his uncovering any "Reds in the Fed" so as not to suffer the same fate. That, in turn, escalates along those lines. The "Red Scare" arrives early as the Soviet's "Terror" becomes more widely known and accepted as factual in the West. This, due to the media's coverage being less forgiving due to the overall political climate having changed.
Thus, when the "Non-Aggression Pact" is revealed, the revulsion against the Soviets is pretty much universal. The Winter War against Finland cements this.
And when Barbarossa kicks off the Soviet efforts at depicting themselves as the victim are met with contempt by the West in general and especially so by the FDR administration.
Hoover, as in Herbert, was very public about how the US should just let the Nazis and the Soviets "bleed themselves white - they deserve each other!" in OTL.
In this ATL, I could see the US happily selling the Soviets all that they could pay for - in gold. No Lend-Lease though. And certainly no US military assistance.
A debatable point would be how much - and how effective - American pressure on the UK to isolate the USSR would be.