FACTBOX-Delegate counts for U.S. presidential candidates

April 28 (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 Nov. 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 1,733

- Hillary Clinton 1,598

REPUBLICANS (number needed for nomination 1,191)

- John McCain 1,266