What does "keep their powder dry on where they stand" mean?

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.