I don't think Republicans have even a remote chance of contesting this seriously, but if we were to, the answer would not be a retread like Michael Steele, but someone newer: Boyd Rutherford comes to mind instantly, and a run here, even if he (as is very likely) loses would be fantastic set-up to another race down the line (probably for Governor in 2022 as Hogan's heir, whether Hogan wins reelection or not).
As for Democrats, O'Malley seems to me to start off as an obvious favorite. Mikulski is the longest-serving woman in the Senate and is well-known for creating an organized bipartisan "club" of Senate women; she may want another woman to replace her. The two most obvious candidates, in that case, seem to be Donna Edwards and Heather Mizeur.
Edwards and Mizeur are both extremely left wing. They'd not be useful for making the Senate more bipartisan and workable. I'd rather see Mayor Rawlings-Blake of any of the three potential women
