Donald Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE on Sunday dismissed a poll that shows rival Hillary Clinton Hillary Diane Rodham ClintonJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Warning signs flash for Lindsey Graham in South Carolina MORE leading him nationally by 12 points.

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"The @ABC poll sample is heavy on Democrats. Very dishonest - why would they do that? Other polls good!" the presumptive Republican presidential nominee tweeted.

The ABC News/Washington Post poll shows Clinton with 51 percent support among likely voters and Trump with 39 percent.

The poll surveyed 1,001 adults reached on cellphones and landlines, according to the Post.

Of those surveyed, 36 percent identify as Democratic, up 3 points from a May version of the poll. Republicans accounted for 24 percent of those polled this month, down 1 percentage point from the May poll. Independents made up 33 percent of those surveyed.

The Post notes that the shift in party identification accounts for less than half of Clinton's gain in the new poll.

The poll comes on the heels of a Reuters/Ipsos poll showing Clinton with a 14-point lead nationally.

An NBC News/Wall Street Journal poll released Sunday gave Clinton a smaller lead, 5 points, over Trump.