From Avion Insurance:
"Rain, snow, sleet, and fog may not affect helicopter performance, but they generally obstruct visibility. These conditions may make take-off and landing more difficult and affect a pilot’s ability to see obstacles that are encountered during flight. Rain, snow, and sleet may also make conditions on the tarmac or landing pad slippery, which can be problematic."
The Secret Service--not the President--determines when it's safe to fly Marine One.
Marine One has all the safety equipment in the world to fly in the rain. There is even a kitchen. He did not want to go. However, that does not bother me, really. What he does today looking forward is what matters.