Hillary Clinton overtook Barack Obama in May, John Edwards and Rudy Giuliani held steady and Ron Paul rocketed from fifth place to first. That's according to Hitwise data showing the ebb and flow of traffic to the official sites of the Democratic and Republican presidential primary candidates.

In a custom report created for ClickZ News, Hitwise measured traffic market share of the candidate sites. The measurement firm found traffic to Democratic candidate sites was top heavy, favoring Clinton's, Obama's and Edwards's sites. HillaryClinton.com garnered nearly a third of visits among Democratic candidate sites in May. BarackObama.com attracted almost 28 percent, and JohnEdwards.com drew 23 percent of visitors to Dem campaign sites last month.

Visits to Republican candidate sites were spread a bit more evenly among the sites tracked. RonPaul2008.com drew a 27 percent share of visitors to Republican candidate sites in May. MittRomney.com attracted almost 21 percent of visits last month, while JohnMccain.com brought in almost 13 percent.

Hitwise defines market share of visits within a site category as the percentage of traffic to a particular site based on its sample of 10 million U.S. Web users.