When Virginia, Maryland, and Washington DC hold primaries on the same day, it is known as the Potomac Primary. Other states in the region may participate in a primary at the same time as well. This primary is sometimes viewed as important in the Presidential race, and it often attracts a great deal of public attention and discussion. It typically falls several weeks after Super Tuesday, the day when a huge group of very disparate American states hold primaries.
For candidates, the Potomac Primary can be a welcome shift after the madness of Super Tuesday. The Super Tuesday primaries require candidates to rapidly travel all over the United States if they want to hold rallies and other events to drum up support. Because the Potomac Primary is held in a group of neighboring states, candidates do not have to deal with grueling travel arrangements, and they can hit more spots in each individual state because the Washington, DC area is relatively small.
Many people have noted that the citizens of Washington, DC, tend to be very politically active, perhaps because this city houses the capital of the United States. In any case, election turnouts in this region are sometimes very good, and it is common for people to get very involved with political campaigns they care about. If a candidate can sweep the Potomac Primary as Barack Obama did in 2008, it can strengthen his or her campaign and political position.
A combined strong performance during the Super Tuesday primaries and the Potomac Primary can sometimes carry the nomination for a political candidate. Even if a candidate doesn't have the nomination sealed up, the Potomac Primary can establish a clear front runner in advance of primaries held in the coming weeks and months. This gives voters a chance to carefully research the candidates, making a decision on the basis of performance in prior primaries and the candidate's record.
Because this region is very active politically, the Potomac Primary can also be a bit of a bellwether to determine how candidates stand up under close scrutiny and analysis. The Washington, DC area also has a very large African American population, and the African American vote is often heavily courted, especially by Democratic candidates. A weak performance in the Potomac Primary can suggest that a candidate may not be able to carry the African American vote in other parts of the country, raising concerns about electability.