McGovern became the most significant figure yet to call for her to stand aside, saying it was virtually impossible to win against Barack Obama.

“The time has come for Democrats to unite to get ready for a tough race this fall against Senator McCain,” McGovern said.

McGovern said he told Bill Clinton that he would support Obama. His decision threatened to set off a widespread defection of superdelegates to Obama that could end Clinton’s race for the White House before the primary season wraps up on June 3. Clinton, however, insisted on Wednesday that she intended to fight on. “I’m staying in this race until there is a nominee,” she said.

Only six contests remain, leaving Clinton with few good options. Tuesday’s results gave Obama his biggest lead over Clinton since voting began. Overall, he now has the support of 1,840 of the 2,025 delegates needed to secure the nomination.

Clinton, however, plans to campaign in Oregon on Thursday, which holds its primary on May 20, and South Dakota, which votes on June 3.