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 U.S. 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 Sen. Barack Obama of Illinois and Sen. 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.

The count does not include all the delegates that will be awarded in Texas, which are still being calculated. The voting there on March 4 had two steps, with two-thirds of the state's 228 delegates awarded based on popular vote and one-third on the outcome of caucuses held after polls closed.

DEMOCRATS (number needed for nomination 2,025)

- Barack Obama 1,400

- Hillary Clinton 1,251

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 contests are winner-take-all when awarding delegates.

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

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

Source of Delegate Count: msnbc.com

(Compiled by Andy Sullivan, editing by Philip Barbara)