The former vice president's victory there, as well as in Missouri, Mississippi and Idaho, dealt a serious blow to Sanders and substantially widened Biden's path to the nomination.

Biden again showed strength with working-class voters and African Americans, who are vital to winning the Democratic nomination. Sanders' narrow hopes for good news rested on North Dakota and Washington state. Washington's primary was too early to call, and because all votes there are cast by mail or by dropping them off in a ballot box, many ballots were marked for candidates who have since dropped out of the race.

The six-state contest marked the first time voters weighed in on the primary since it effectively narrowed to a two-person race between Sanders and Biden. And the first four states on Tuesday went to Biden, a dramatic reversal for a campaign that appeared on the brink of collapse just two weeks ago.