Being curious, I looked into it more and there's a Wikipedia article about it:
District of Columbia retrocession
Maybe a lot of you knew this, but I did not - DC was originally a 100 square mile block of land from both Maryland and Virginia, on both sides of the Potomac. In 1846 the former Virginia side (Alexandria County) successfully petitioned to have their side of DC transferred back to Virginia.
As for the "return to Maryland" side:
Quote:
Most residents of Maryland and Washington, D.C. do not support retrocession. A 1994 study showed that only 25% of suburban residents polled endorsed retrocession to Maryland, and that number dropped to 19% among District residents. District residents’ dislike was confirmed in a 2000 George Washington University study when only 21% of those polled supported the option of retrocession.[26] A 2016 poll of Maryland residents showed that only 28% supported annexing Washington, D.C. while 44% were opposed.[27]
Republican legislators have introduced bills to cede DC to Maryland that Democrats have opposed - article doesn't say why, but I assume it's a poison pill to stop DC from getting statehood and thus Senate seats.