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First Read: Clinton's challenge is winning over Sanders voters

Can Hillary Clinton win over Bernie Sanders' supporters in what has become an increasingly asymmetric Democratic contest?

First Read is a morning briefing from Meet the Press and the NBC Political Unit on the day's most important political stories and why they matter.

Hillary's challenge: winning over Sanders voters

Two forces are at play in our new NBC/WSJ poll showing Hillary Clinton's lead over Donald Trump shrinking to just three points, 46%-43%. One, Republicans have rallied around Trump since he became his party's presumptive nominee. They're now backing him over Clinton by an 86%-to-6% margin -- up from 72% to 13% a month ago.

Two, the still-ongoing Democratic primary race has taken a toll on Clinton, who is all but certain to be her party's nominee. While Democrats are backing Clinton by an 83%-to-9% clip in the poll, just 66% of Democratic primary voters preferring Sanders support Clinton in a matchup against Trump (compared with 88% of Clinton primary voters who favor Sanders in a hypothetical general-election contest). What's more, Clinton's fav/unfav rating among Democratic voters supporting Sanders is 38% positive, 41% negative (-3); Sanders' rating among Clinton supporters is 54% positive, 23% negative (+31).

So this is Clinton's challenge: Can she win over Sanders' supporters in what has become an increasingly asymmetric Democratic contest -- with Clinton supporters liking Sanders, but with Sanders supporters disliking Clinton? It could be the difference between Democrats holding the advantage in November, or an incredibly close general-election contest.