FACTBOX - Delegate counts for U.S. presidential candidates

(Reuters) - Delegates at national party conventions in August and September will select 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 continues between Sens. Barack Obama of Illinois and Hillary Clinton of New York.

The Democratic Party voted on Saturday to seat half of the disputed Michigan and Florida delegations, moving the number to clinch the nomination to 2,118 delegates from 2,026.

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 2,118)

- Barack Obama 2,067

- Hillary Clinton 1,906

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.

There are 31 Democratic delegates left to be chosen in the remaining two contests -- South Dakota and Montana on Tuesday.

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.

(Compiled by Andy Sullivan in Washington; Editing by Chris Wilson)