Democratic US presidential candidate Hillary Clinton speaks at the US Conference of Mayors 84th Annual Meeting in Indianapolis, Indiana United States, June 26, 2016. [Photo/Agencies]

WASHINGTON - US presumptive Democratic nominee Hillary Clinton is holding a 12-point lead over her Republican rival Donald Trump as support for the latter tumbled in the past month, found a poll released Sunday.

Clinton enjoys 51 percent of support among Americans, up by seven points from May, compared to Trump's support at 39 percent, which is down by seven points, according to the Washington Post-ABC News poll.

This is Clinton's biggest lead over Trump since last fall and is a dramatic reversal from the May poll, in which Trump led Clinton slightly by 46 percent to 44 percent.

The poll discovered that 56 percent of American voters believe Trump stands against their beliefs, while 64 percent say Trump does not have the necessary credentials to be president.

Clinton's lead over Trump narrows to 10 points by 47 percent to 37 percent, when the two third-party presidential candidates were mentioned in the poll. Libertarian Party's Gary Johnson gains 7 percent of support, while Green Party's Jill Stein garners only 3 percent, according to the poll.

The controversial comments on women, minorities and Muslims made by Trump, a brash New York billionaire, may have contributed to his recent slide in the poll, as the majority of American voters regard them as biased.

Overall, 66 percent of Americans think Trump's remarks as "unfairly biased," while only 29 percent say they are "not unfairly biased."

Even among the Republicans, 41 percent think Trump's remarks are "unfairly biased," while 53 percent say they are "not unfairly biased."

Trump has been criticized recently by publicly attacking a Mexcian-American judge overseeing the Trump University fraud case. The poll found that 68 percent of Americans regard Trump' s criticism of Judge Gonzalo Curiel as racist and 85 percent say they are inappropriate.