The Democrats' long primary campaign ended Tuesday in historic fashion, with Sen. Barack Obama becoming the first African American to win enough delegates to clinch a major party's presidential nomination.

The race is thus narrowed to the Illinois senator and the putative Republican nominee, Sen. John McCain of Arizona. Both have begun sharpening their positions on the issues -- and their teeth on each other.

Their differences assure that Americans will have a real choice in November. Mr. Obama favors ending the Iraq war, while Mr. McCain says he would continue the U.S. presence for a hundred years if necessary. Mr. McCain will have to separate himself from the struggling U.S. economy, a result of policies by the current Republican president, George W. Bush.

Mr. Obama faces a challenge in the aftermath of the tough campaign Sen. Hillary Rodham Clinton of New York waged against him after the Democratic field was thinned. His speech Tuesday night included generous praise for her. Her speech was a reiteration of why the voters should have chosen her instead of him.

Mr. McCain and Mr. Obama will turn to choosing their running mates between now and the party conventions. Some Democrats say Mrs. Clinton should be on Mr. Obama's short list, based on the presumption that she can deliver to the ticket the votes and financial contributions of those who supported her in the primaries. But this assumption may be false.

Her campaign ended in the red and she does not control the votes of those who backed her in the primaries. Mr. Obama also could have several reasons for not choosing her as his running mate, including her use of the race card against him, the fact that a Clinton on the ticket would not represent change and the chance that Mrs. Clinton might be disruptive as No. 2.

The choice for voters between Mr. McCain and Mr. Obama will become clearer as their debate unfolds. Americans start from the point that Mr. Obama is a history-making nominee, while Mr. McCain resembles the more standard model of presidential candidate.

Voters will also see a generation of difference: Mr. McCain will be 72 on Election Day and Mr. Obama, 47.

For many Americans Tuesday night, there was some relief that the choice is now down to two. How the voters decide in November will make its own history.