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 has taken a commanding lead in the Republican race, while the Democratic contest remains close.

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 (total needed for nomination 2,025)

- Barack Obama 943

- Hillary Clinton 895

REPUBLICANS (number needed for nomination 1,191)

- John McCain 724

- Mike Huckabee 234

- Ron Paul 14

HOW DELEGATES ARE AWARDED

Democrats distribute delegates in proportion to their 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 last week when his chief rival dropped out. But former Arkansas Gov. Huckabee remains in the race and beat McCain on Saturday in Louisiana and Kansas and ran a very close second in Washington state.

In addition to those elected state by state, a certain number of delegates at the conventions are set aside to be 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.