Recent polling from Abt SRBI has LG Anthony G. Brown (D) leading 32-15 over State AG Douglas Gansler (D), while Heather Mizeur (D) has 9%.
I know a poll for the Sun had something like 32-14-10. Brown and Gansler have both come out with advertising recently, too. From my (completely anecdotal) experiences phonebanking for the Mizeur campaign, I can tell you most voters are undecided. Brown is the clear frontrunner and has almost all of the establishment endorsements locked down. He's up big in PG, the City, and Howard/the rest of Maryland. Gansler has significant support in MoCo, as does Mizeur. Brown has a lot of money and can raise money during the legislative session, due to a stupid loophole (his Lt. Gov. pick, Ken Ulman, is a County Executive (for Howard, my home county)), as can Mizeur (because she's doing public financing). I'd say most support for Brown is pretty soft right now, and I'd give the chance he wins at something like 70%.