Rand Paul Up, Hillary Clinton Down in 2016 US Presidential Poll

Democrat Hillary Clinton has slipped against leading 2016 Republican candidates in Colorado, Iowa and Virginia, according to a poll released on Thursday that cited damage from the furor over the former secretary of state's emails.

The Quinnipiac University Swing State Poll also showed Clinton in a close race with US Senator Rand Paul, the Kentucky Republican who declared his candidacy on Tuesday.

Clinton, who is expected to announce her White House bid this month, is tied with all the Republican candidates in Colorado and almost all of them in the early voting state of Iowa, the poll said.

"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" since the last swing state survey on February 18, pollster Peter Brown said in a statement.

He attributed the drop to the controversy that erupted in March over Clinton's use of personal email for work when she was America's top diplomat. Republicans have raised the prospect of congressional hearings on the issue.

Paul led Clinton by 44 per cent to 41 per cent in Colorado and 43 per cent to 42 percent in Iowa, the Quinnipiac poll showed. In Virginia, Clinton led 47 per cent to 43 per cent.

"These numbers are a boost for US Senator Rand Paul of Kentucky as he formally launches his campaign," Brown said.

The poll cited a bright spot for Clinton in Virginia, where she led all Republicans, including former Florida Governor Jeb Bush, whom she led with 47 per cent to 40 percent, compared to a 42-42 per cent tie in February.

The telephone poll of 2,803 likely voters was conducted from March 29 through April 7 and had an error margin of 3.2 per centage points in Iowa and Virginia and 3.3 points in Colorado.