Bernie Sanders reacts to cheering supporters during a campaign rally in Sacramento, California, May 9, 2016.

On the Republican side, meanwhile, Donald Trump became the party's presumptive nominee when his final two opponents suspended their campaigns last week.

Voters in West Virginia took to the polls as the primary season appeared to draw to a close. Although most pundits say former Secretary of State Hillary Clinton has essentially secured the nomination because of delegate math, Sanders has pledged to continue his campaign.

Sanders had appeared positioned for a victory in West Virginia: Pre-primary polls had indicated he held a lead of a few percentage points over Clinton.



But NBC reported that 40 percent of Sanders' West Virginia voters said they would actually support Trump over the senator in a general election match-up — potentially suggesting that some independents' party affiliations for the day were more about the state's Democratic governor primary than their presidential preference. (There was no GOP gubernatorial contest on Tuesday.)

Clinton has attempted to pivot to the general election, but she has been forced in recent weeks to defend her likely nomination from Sanders' continued primary strength.

In fact, Sanders is positioned to have a good showing in May, which will also see Oregon voters voice their Democratic preference.