I've come across the "come + X" construct in a passage of a New York Times article. Here it is (emphasis added):
Politicians like to keep the fiscal levers in their hands come election time
What kind of clause does it introduce? Plus, how common is it? Is it typically American? Formal or informal? Are there any other instances where it is used, besides "come election time," which seems to me a set phrase? Lastly, which mood is that "come"?