Hillary Clinton's standing has taken a tumble in key swing states after an investigation into her controversial email practices brought her biggest weakness with voters into the forefront.

A new Quinnipiac University survey covering swing states Florida, Ohio and Pennsylvania shows presumptive Republican nominee Donald Trump benefiting from Clinton's tough headlines, even as his own support ticked up just marginally.

In Florida, Trump leads 42 percent to 39 percent – narrowly within the poll's margin of error, according to results released Wednesday. But the tight race marks a major swing from a previous June 21 survey, when Clinton led 47 percent to 39 percent.

That poll, along with the one from JMC Analytics showing Trump with a 5-point lead, eliminated what had been Clinton's 4-point advantage as recently as Saturday in RealClearPolitics' rolling aggregate of surveys in the Sunshine State.

Quinnipiac also found the candidates in a statistical dead heat in Ohio – where they are tied at 41 percent – and Pennsylvania, where Trump leads 43 percent to 41 percent.

"Donald Trump enters the Republican convention on a small roll in the three most important swing states in the country. He has wiped out Hillary Clinton's lead in Florida; is on the upside of too-close-to-call races in Florida and Pennsylvania and is locked in a dead heat in Ohio," Peter Brown, assistant director of the Quinnipiac University Poll, said in a news release.

"While there is no definite link between Clinton's drop in Florida and the U.S. Justice Department decision not to prosecute her for her handling of emails, she has lost ground to Trump on questions which measure moral standards and honesty," Brown said.

Polls in other swing states show a similarly competitive race.

In Iowa, a Monmouth University poll out Tuesday gave Trump a 2-point edge over Clinton, while other Hawkeye State surveys late last month showed the former secretary of state in the lead. In Nevada, where a GQR survey gave Trump a narrow lead several weeks ago, Monmouth found Clinton with a 4-point edge in its poll released Monday.

In Colorado, meanwhile, a CBS News/YouGov tracking poll showed just 1 point separating Clinton from Trump in late June, but a Harper Polling survey last week saw Clinton open up a 7-point advantage.

"We know the battlegrounds are going to be close til[l] the end," Brian Fallon, Clinton's national press secretary, said in a tweet Wednesday morning. "That's why we need to keep working so hard. Trump is a serious danger, folks."

Nationally, the race has been relatively stable, with Clinton maintaining a single-digit lead in RealClearPolitics' average .

Clinton's email controversy returned to dominate headlines last week, when FBI Director James Comey said his office would not recommend criminal charges related to her use of a private email server while secretary of state. But Comey also called Clinton and her staffers "extremely careless" in the handling of classified material.

For his part, Trump has baffled politicos of late by grabbing the spotlight in ways that at times have reflected poorly on him. He praised former Iraqi President Saddam Hussein and defended a tweet that was seen as anti-Semitic, rather than letting Clinton's negative headlines dominate the news cycle.

Yet more recently, Trump has managed to avoid similar controversies, keeping voters largely focused on Clinton's lingering trustworthiness issues as he prepares for the Republican National Convention and vets vice presidential candidates .