What does "keep their powder dry on where they stand" mean?
Before the Battle of Dunbar in 1650, Oliver Cromwell famously told his troops to "Trust in God and keep your powder dry" (gunpowder attracts moisture very easily). So the phrase means 'make sure you are suitably prepared', not 'keep quiet' as the Senator's letter seems to imply. However, perhaps he meant that keeping their opinion secret for now was a preparation for later events.