FACTBOX: Delegate counts for presidential candidates

(Reuters) - Delegates at national party conventions in August and September will be the key to selecting the Democratic and Republican candidates who will face off in the presidential election on November 4.

Voters choose the delegates state by state.

Sen. John McCain of Arizona has won enough delegates to become the Republican nominee, while the Democratic contest remains close between Sens. Barack Obama of Illinois and Hillary Clinton of New York.

Here are the total number of delegates awarded so far in nominating contests to the leading candidates, as estimated by MSNBC. Other news organizations may have reached different estimates.

DEMOCRATS (number needed for nomination 2,025)

- Barack Obama 1,728

- Hillary Clinton 1,596

REPUBLICANS (number needed for nomination 1,191)

- John McCain 1,266

HOW DELEGATES ARE AWARDED

Democrats distribute delegates in proportion to candidates' votes statewide and in individual congressional districts. That means candidates can come away with big chunks of delegates even in states they lose.

In contrast, most Republican states award their delegates on a winner-take-all basis.

In addition to those elected state by state, a certain number of delegates at the conventions are set aside for elected officials and other leading party figures.

These "superdelegates" are not committed to a particular candidate and can back anyone they choose.

The number of superdelegates fluctuates at times, if someone resigns or if a new official with superdelegate status is elected somewhere in the country. The number just increased by one because there was a special election in which a Democrat was elected. That Democrat counts as a new superdelegate.

(Compiled by Deborah Charles in Washington, editing by Jackie Frank)