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In a four-way race that included Libertarian nominee Gary Johnson and Green Party nominee Jill Stein, as well as Republican nominee Donald Trump, Clinton hit 50% support.
It was just the second four-way poll posted to RealClearPolitics that showed Clinton with 50% support.
The other instance was in early August, when Trump experienced a near-identical collapse in the poll following a series of controversies. That poll was also conducted by Monmouth.
In Monday's poll, Clinton was ahead of Trump 50% to 38% among likely voters.
"Clinton has increased her lead among all registered voters, but the main difference between this month and last month is that her supporters have become more enthusiastic, and thus more likely to turn out while Trump backers have become less likely to vote," said Patrick Murray, director of the independent Monmouth University Polling Institute.
Trump's polling numbers deteriorated following the release of a tape from 2005 earlier this month in which Trump made a series of sexually explicit remarks about women.
Following the release of the tape, a score of women proceeded to make accusations against Trump of having made unwanted sexual advances on them in the past.
In the RCP four-way polling average, Clinton leads Trump by 6.9 points - her largest lead of the general election cycle.
RealClearPolitics
Business Insider's most recent Electoral College projection found that Clinton, if the election were to be held as of late last week, would most likely have enough electoral votes to win the presidency.
Business Insider/Andy Kiersz, data from RealClearPolitics and Washington Post/SurveyMonkey