AP/Pablo Martinez Monsivais
Former Secretary of State Hillary Clinton.
Quinnipiac surveyed three crucial swing states - Colorado, Iowa, and Virginia - and found Clinton's lead slipping against her possible Republican rivals. Peter Brown, assistant director of the polling outfit, said the shift is because voters increasingly see Clinton as a dishonest candidate.
"Clinton's lead is wilting against leading Republican presidential candidates," Quinnipiac said in its write-up of the survey. "In head-to-head matchups, every Republican candidate effectively ties her in Colorado and almost all Republicans effectively tie her in Iowa."
Sen. Rand Paul (R-Kentucky), who launched his campaign for president this week, performed particularly well against Clinton in the poll. In Colorado, Paul led Clinton 44% to 41%, and in Iowa, he narrowly led her 43% to 42%.
However, Brown said Clinton should be concerned that all of her potential Republican foes appear to be gaining traction against her.
"These numbers are a boost for US Sen. Rand Paul of Kentucky as he formally launches his campaign," he said in a statement. "Ominous for Hillary Clinton is the broad scope of the movement today compared to her showing in Quinnipiac University's mid-February survey. It isn't just one or two Republicans who are stepping up; it's virtually the entire GOP field that is running better against her."
Brown attributed Clinton's drop in the polls to the recent controversy over her email use as secretary of state. At the start of last month, The New York Times reported Clinton exclusively used her personal email for official government business, which allegedly broke federal guidelines and could have placed sensitive communications at risk. Facing a barrage of criticism, Clinton eventually said she turned over about 30,o00 work-related emails to the State Department. However, she raised eyebrows by revealing she deleted the other 30,000 "personal" messages on her server.
"It is difficult to see Secretary Clinton's slippage as anything other than a further toll on her image from the furor over her email," Brown said.
AP Hillary Clinton discussing her emails at the United Nations. 
"In all three of these states, more, and in Colorado many more, registered voters say she is not honest and trustworthy," Brown added. "Voters do think she is a strong leader - a key metric - but unless she can change the honesty perception, running as a competent but dishonest candidate has serious potential problems."
In his own analysis of the poll, Veteran Politico reporter Mike Allen agreed that the coverage of Clinton's email controversy has taken a toll on her likely presidential campaign. Accordingly, he said it is likely Republicans will increasingly attack Clinton for being a "liar."
"So now we know why, as Rand Paul revealed to Politico in a post-announcement interview, he plans to make a major push on Clinton character, focusing on emails and Clinton Foundation finances," Allen wrote in his Thursday morning newsletter. "We can see from this poll that the coverage is killing Clinton, putting her even with clowns like Huckabee in swing states (tied 41-41 in Colo.) because so many people consider her a liar. GOP focus groups show this vulnerability. So even more money and more groups will focus on her scandals."
However, the Quinnipiac poll also had some good news for Clinton: She's holding onto her lead in Virginia. The state used to be solidly Republican but voted twice for President Barack Obama. In the survey, Clinton led Paul 47% to 43% in the state and beat other Republicans by even wider margins.
"Of the three states tested, Virginia seems to be the friendliest toward Hillary Clinton, perhaps a continuing reflection of the Old Dominion's leftward drift over the past decade," Brown said. "Only yesterday, it seemed, it was deeply red."
Below is the full release on the Quinnipiac poll: