I imagine he saw the study that 50% of Republican men wouldn't take the vaccine - https://www.thelily.com/49-of-republ...e-their-minds/
And did the mental calculus knowing that men make up around 60% of the Republican party and got worried about what potentially losing upwards of 20-30% of national Republican registered voters would do to their electoral chances across the board.
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See my link above, because he's worried that Republican men refusing the vaccine will literally kill the party, not just themselves.