According to the Associated Press delegate count, John McCain has clinched the Republican presidential nomination.

McCain first added Vermont, then Ohio to his column, clinching the GOP nomination. President Bush has invited the Arizona senator to the White House tomorrow for a show of support.

Following McCain's victory, candidate Mike Huckabee dropped out of the presidential race. Huckabee said called McCain to send his congratulations and hopes his followers will continue to back the party.

On the Democratic side, Barack Obama and Hillary Rodham Clinton are dueling in competitive contests in Ohio, Texas and Rhode Island. Earlier this evening, Obama defeated Clinton in the Vermont primary. It's his 12th straight win over the former first lady.

Clinton is hoping to rebound later in the night in the other three states. Obama has taken an early lead in Texas based almost entirely on votes cast before primary day. The Ohio count is being delayed by heavy voting that kept some polls in Sandusky and Cleveland open for 90 minutes past the scheduled 7:30 p.m. closing time.

Copyright 2008 by The Associated Press. All Rights Reserved.

Drew Sandholm contributed to this report.