I have searched here and googled with no result. I can't remember the name of the author. I think it begins with a B. I can't remember the name of any of the books, nor the name of the series. It's something like Day Zero or Day One but googling that is not helpful. I read these books in about 2010. I am sure the series is not more than 20 years old.
In these books, a radical underground group develops a bacteria that eats all the fuels and plastics. They release it on the world with the intention of killing most of the world's people, to arrive at a "sustainable" population of about, I think, 100 million. Civilization comes to a halt. Any technology that escapes the "plague," as I think it's called, has rocks thrown at it by a catapult on the moon. Steam power works. The US government falls apart and has two rival governments, one in Colorado Springs and one in Atlanta, I think. Eventually the two governments agree to disagree on how to run the former USA. There is quite a bit of intrigue and politics.