The last sentence in the Washington Post's Battle over the replacement of Ruth Bader Ginsburg rages as tributes to late justice pour in says:
Senate Majority Leader Mitch McConnell privately told his members in a letter circulated Friday night to keep their powder dry on where they stand on proceeding with a confirmation fight this year.
In this context the author of the letter needs the recipients to behave in a certain way in order to win what may be a long "fight" and has offered this as a a first set of instructions. Others will follow.
This might be something along the lines of keep quiet, or stand firm, or perhaps remain in a state of readiness to respond to further instructions, but I can't tell which.