Hillary Clinton's lead among Democrats is shrinking, dropping nine points since April, according to a new Monmouth University Poll released today.

Clinton still holds an enormous lead over her nearest competitor, Independent Vermont Senator Bernie Sanders, with 51 percent of the vote to Sanders' 17 percent, but just last month, Clinton led 57 percent to 12 percent in the same poll.

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Increasingly, Democrats are supportive of a Democratic challenger to Clinton -- with 53 percent agreeing that it would "be better if there was an active primary challenge," up from 48 percent last December. But Clinton continues to have the best favorability ratings among her Democratic rivals -- 74 percent favorable to 17 percent unfavorable.

15 percent of voters are undecided, besting the 13 percent of voters who indicated they are still holding out hope for a Joe Biden run. Biden holds the second highest favorability ratings in the Democratic field with 67 percent of Democratic voters viewing him favorably.

For his part, Sen. Sanders' favorability ratings have surged as more Democratic voters are introduced to his campaign. 36 precent of respondents held a favorable view of the self-described democratic socialist, up from 29 percent last month.

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The results in this poll release are based on a subsample of 357 registered voters who identify themselves as Democrats or lean toward the Democratic Party from a larger, national random sample of 1,001 adults age 18 and older. The poll was conducted July 9 to 12, 2015 with a margin of error of plus or minus 5.2 percentage points.