get on (one's) hobby-horse

To frequently or incessantly talk or complain about a subject, topic, or issue in which one is excessively interested. "Hobby-horse" can also be spelled as a single word." Ah, here we go again. Once Grandpa gets on his hobby-horse about the government, there's no stopping him! Uh oh, sounds like Janet is getting on her hobbyhorse again about the need for healthcare reform. If Joe gets on his hobby-horse about public safety, the Dean of Students will be listening to that for a long time.