Part of that may be thanks to former Sanders supporters, who appear to be coalescing around the presumptive Democratic nominee.
According to the poll, just 8% of respondents who backed Sanders in the primaries said they will support Trump over Clinton in the general election. That was down from 20% percent last month.
PHOTOS: Donald Trump's 2016 campaign
Over the last several months, Trump has repeatedly parroted some of Sanders' rhetorical attacks on Clinton over the former secretary of state's ties to the financial industry and political elites.
During a Wednesday speech, Trump made a direct appeal to the senator's supporters, highlighting the real-estate magnate's skepticism of international trade deals.
"The insiders wrote the rules of the game to keep themselves in power and in the money," Trump said. "That's why we're asking Bernie Sanders' voters to join our movement — so together we can fix the system for all Americans. Importantly, this includes fixing all of our many disastrous trade deals. Because it's not just the political system that's rigged. It's the whole economy."
Trump's attempt to reach disgruntled Democrats unsatisfied with Clinton isn't unfounded — there's plenty of discontent with the presumptive Democratic nominee among Sanders' supporters.
At a speech in New York on Thursday, a supporter waved a "Never Hillary" sign several feet away from where Sanders stood onstage, while the senator was repeatedly interrupted by calls for him to run against both Clinton and Trump.
And while Sanders has said that he plans on voting for Clinton in the general election, he has withheld an official endorsement, continuing to pressure her to embrace more progressive policy proposals.
"I'm going to do everything I can to defeat Donald Trump. But a lot of that responsibility about winning the American people over to her side is going to rest with Secretary Clinton," Sanders said during a CNN interview Sunday. "We got something like 12 to 13 million votes. And those people voted for me, I believe, because they said it is time to have a president to have to guts to stand up to the big money interests."
He added: "We are saying to Secretary Clinton: Make it clear which side you're on."