FACTBOX: Tuesday's "Potomac Primary" presidential contests

(Reuters) - Maryland, Virginia and the District of Columbia, the U.S. capital, hold nominating contests on Tuesday, part of the state-by-state process to choose the Republican and Democratic candidates for the November presidential election.

Following are a few facts about the "Potomac Primary," nicknamed after the river that splits the region:

* Voters in the U.S. capital city will have a rare chance on Tuesday to affect national politics. The District of Columbia has no voting representation in Congress, and the city's overwhelmingly Democratic tilt means presidential candidates rarely bother to campaign there in general elections.

* In the Democratic contests, 83 delegates are at stake in Virginia, 70 in Maryland and 15 in the District. The primaries are not winner-take-all, and the delegates will be divided between New York Sen. Hillary Clinton and Illinois Sen. Barack Obama based on how they do statewide and in individual congressional districts.

* In the Republican contests, 63 delegates are at stake in Virginia, 37 in Maryland and 19 in the District. The winner of each contest gets all of the delegates, which could allow front-runner John McCain, an Arizona senator, to pull farther ahead of his last remaining rival, former Arkansas Gov. Mike Huckabee.

* Blacks make up 57 percent of the population in the District, 30 percent in Maryland and 20 percent in Virginia -- above the national average of 13 percent. Blacks have heavily favored Obama, who would be the first black president, over Clinton in Democratic state contests so far.

* Voting ends at 7 p.m. EST in Virginia and at 8 p.m. EST in Maryland and the District.

* In Maryland and the District voters must register ahead of time and select a party. They can only vote for candidates running from the party with which they registered. Since voters in Virginia do not register by political party, any registered voter may choose either the Democratic or Republican ballot.

Source: National Association of Secretaries of State, U.S. Census Bureau

(Reporting by Andy Sullivan; editing by Mohammad Zargham)