President George W. Bush

U.S. President George Bush is fundraising for presumptive Republican presidential nominee John McCain, who generally has distanced himself from the unpopular president.

Mr. Bush is appearing with McCain at a fundraiser in the city of Phoenix, in the senator's home state of Arizona Tuesday, in an event that is closed to the press.

While McCain has supported President Bush on the war in Iraq, he has differed with the president on other issues, such as global warming - criticizing Mr. Bush for not doing enough to combat the problem.

Republican strategists have expressed concern that the president's low approval rating could hurt the Republican nominee in the general election.

Barack Obama

The frontrunner for the Democratic Party's nomination, Barack Obama, has promoted a campaign for change by pointing out McCain's similarities with the president. He has said McCain represents a third term for President Bush.

Obama and his Democratic rival, Hillary Clinton, have both promised to end the war in Iraq, highlighting a major difference in policy from McCain and President Bush.

Clinton trails Obama in the number of delegates needed to secure the party's nomination. She has rejected calls to step aside, vowing to continue her campaign through the final three primary elections.

The U.S. Commonwealth of Puerto Rico holds the next primary on Sunday. The final two contests are in the western states of Montana and South Dakota on June 3.

While Obama leads the delegate count, it is unlikely either he or Clinton will have enough pledged delegates to secure the nomination.

That means the contest will likely be decided by so-called superdelegates - Democratic officials and elected office-holders who are free to vote for either candidate - at the party's nominating convention in August.

Some information for this report was provided by AP , Bloomberg and Reuters.