Deteriorate, worsen, as in Ever since the recession began, the business has been going downhill. The figurative use of downhill for “decline” dates from the mid-1800s. Also see downhill all the way.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.