Facing dismal poll ratings and insults from right-wingers within his own flagging conservative movement, British Prime Minister John Major abruptly resigned as Conservative Party chairman to force a leadership election next month. "I am no longer prepared to tolerate the present situation," said a bitter Major, who must resign as prime minister if he loses the July 4 vote. "In short, it is time to put up or shut up."TIME London bureau chief Barry Hillenbrandsays Major, considered "a gray leader and not very good," is caught in a political vise. Critics on the right increasingly see him as a weak bulwark against Britain's closer integration with the 15-nation European Union. At the same time, as Britain's political center has drifted leftward, "the party is disintegrating in front of him." That left today's gambit, which Hillenbrand says has already drawn a number of the faithful to his cause, though opponents within his party still have time to gather and unseat him. Whatever the outcome, Hillenbrand says, the move does nothing to contain the mounting popularity of the Labour Party, which will pit an increasingly popular Tony Blair against Major -- or another Conservative Party candidate -- in 1997.