FACTBOX - Delegate counts for U.S. 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.

The field of candidates has narrowed and Sen. John McCain of Arizona has taken a commanding lead in the Republican race, while the Democratic contest remains close between Sen. Barack Obama of Illinois and Sen. Hillary Clinton of New York.

Here are the total numbers 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 1,194

- Hillary Clinton 1,037

REPUBLICANS (number needed for nomination 1,191)

- John McCain 930

- Mike Huckabee 248

- Ron Paul 14

HOW DELEGATES ARE AWARDED

Democrats distribute delegates in proportion to candidates' vote 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. McCain became the likely Republican nominee when his chief rival dropped out. But former Arkansas Gov. Huckabee remains in the race.

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.