Hillary Clinton's presidential campaign has opened up a sizeable lead over Donald Trump in Pennsylvania, but is locked in tight races with the Republican nominee in two other key swing states, according to a Quinnipiac University poll released Tuesday.

In a two-way race, the poll found that Clinton leads Trump by 10 points in Pennsylvania. In Florida, the candidates are virtually tied, with Clinton holding a 46-45 percent lead. In Ohio, Trump trails Clinton 45-49 percent.

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Clinton's margins tightened when pollsters presented likely voters with third-party options. Clinton leads Trump 48-39 percent in Pennsylvania in a four-way race, the poll found, with Libertarian Party nominee Gary Johnson and Green Party nominee Jill Stein polling at seven and three percent, respectively. But in Florida, Trump and Clinton are tied at 43 percent; and in Ohio, Clinton holds a slim lead of 44-42 percent when likely voters were asked to choose among the major- and third-party candidates.

In an election that pits two of the most disliked presidential nominees in modern history against one another, many of those polled reported that their decision was based less on their support of one candidate than out of their disdain for the alternative. Over 40 percent of Clinton voters in all three states said the main reason they supported Clinton was their opposition of Donald Trump, and over half of Trump supporters in all three states said they were voting in opposition to Clinton.

Quinnipiac found that gender is a significant delineating factor in each of the three crucial battleground states. Clinton holds a significant lead among women in all three states, ranging from a 13 percent margin in Florida to a whopping 23 percent gap in Pennsylvania. More men support Trump in each state, but his leads are more modest, ranging from five percent to 12 percent.

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From July 30 to August 7, Quinnipiac surveyed 1,056 likely Florida voters, with a margin of error of plus or minus 3 percentage points; 812 likely Ohio voters, with a margin of error of plus or minus 3.4 percentage points; and 815 likely Pennsylvania voters with a margin of error of plus or minus 3.4 percentage points.

Another poll conducted by NBC News and SurveyMonkey, also released Tuesday, found that Clinton holds a 10 point lead on Trump nationally in a two-way race, and a six point lead in a four-way contest.