Martin O'Malley, the Mayor of Baltimore, and Doug Duncan, the Montgomery County Executive, emerged as the two Democratic candidates for Governor in late 2005. Early polling indicated that O'Malley would have the upper edge in both the Democratic primary and the general election, with a solid lead over Duncan in the primary and a several point lead over Ehrlich in the general.
As the gubernatorial campaign intensified, Doug Duncan dropped out of the race, citing the fact that he had been diagnosed with clinical depression. In the same announcement, Duncan threw his support behind O’Malley and declined to seek another office in the fall. O'Malley then became the presumed Democratic nominee for Governor, as no other candidate opposed him in the primary election.