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Bernie Sanders has regained some momentum in the race for the Democratic presidential nomination.

(The Associated Press)

Vermont Sen. Bernie Sanders is trying to claw his way back.

After former Secretary of State Hillary Clinton ran off a series of victories in Northeast states last month, politics watchers all agreed that Sanders' long-shot campaign for the Democratic presidential nomination had finally run aground.

But Sanders refused to accept the conventional wisdom. And last week he kept his hopes alive by pulling off an upset win in the Indiana primary. He did it again today, scoring an impressive win in West Virginia. Sanders has 50.8 percent of the vote in the Mountain State to Clinton's 38.2 percent, with nearly half of precincts reporting.

To be sure, Sanders remains a long shot for the nomination. His net gain from the West Virginia win likely will be only a couple of pledged delegates. With just a month left in the primary season, he has to win nearly 70 percent of the remaining bound delegates to shove Clinton out of the way and make a serious grab for the scepter. And even if he manages to win out by such inconceivable margins, there's still no guarantee. There are those 500-plus superdelegates -- party insiders guided by their own preference, not the will of the people -- who have committed themselves to Clinton.

Presumptive Republican presidential nominee Donald Trump, who is struggling to unify his party, is happy to see the Democratic battle stretch out. He mocked Clinton this week, saying she "can't close the deal" with her own party's voters, and heralded new polls that suggested he could beat her in November in key battleground states such as Ohio and Pennsylvania. Sanders, meanwhile, crows that he polls better than Clinton in head-to-head match-ups against Trump.

Next week, Oregon and Kentucky hold primaries. Famously progressive Oregon has long been expected to be a strong state for Sanders, but a new poll from DHM Research shows Clinton winning it 48 percent to 33 percent. Find out more about the poll.

Regardless of what happens next week, Clinton will remain on track to be the Democrats' standard-bearer in the general election. Still, she needs to win California on June 7 to effectively "close the deal" -- and to give herself much-needed momentum going into the party's national convention.

-- Douglas Perry