on someone's side
In support of someone's views or interests, as in I'm glad you're on my side in this debate, or With the Canadians on our side, we should be able to persuade the Mexicans of a North American policy. [1300s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.