In Ohio, a "must-win" state for Clinton's campaign, she led Obama by 55 percent to 43 percent, according to the results of 88 percent of the precincts, a much wider lead than indicated by pre-primary polls.

Democratic presidential candidate Senator Hillary Clinton (D-NY) speaks at a rally in Dallas, Texas March 1, 2008.(Xinhua/Reuters Photo)

"For everyone here in Ohio and across America who has ever been counted out but refused to be knocked out, for everyone who has stumbled but stood right back up, and for everyone who works hard and never gives up," she told a rally in Ohio.

Her popularity in the state was buttressed by a large number of blue-collar workers, Catholics, elderly and white women voters, political analysts said.

According to CNN exit polls, Clinton gained 54 percent of female votes in Ohio while Illinois Senator Obama took a lead among male voters at 52 percent.