The party was dismissed by the establishment as too liberal, too radical, even revolutionary. But it scored a stunning victory in Thailand’s election last year, as millions of voters delivered a rebuke to the country’s monarchy, its military and its moneyed elite.

       The old guard reacted swiftly, moving to crush its most formidable challenger in decades. Conservative politicians prevented the Move Forward Party’s leader from becoming prime minister and engineered a coalition that kept the party out of power.

       On Wednesday, their quest to reverse the election results seemed complete: Move Forward was disbanded by Thailand’s Constitutional Court over charges that the party’s proposals to water down a stringent royal defamation law were an attempt to overthrow the monarchy. The nine-member court in Bangkok ruled unanimously.

       It also barred from politics for a decade nearly a dozen party operatives, including Pita Limjaroenrat, its former leader, and four other elected members. The other 140 or so Move Forward members of Parliament were given two months to join another party. They are expected to remain the core of the opposition in the 500-seat legislature.

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       At a news conference after the decision, Mr. Pita said that his colleagues plan to unveil a new party on Friday — one that he will not be a part of — and strive to lead an opposition that is not squashed by the establishment.

       “We’ll try our best to break that cycle so that Thailand becomes fully democratic,” he said.

       In Thailand, which has had a dozen coups in the past century, elections are often meant to provide a junta with a veneer of democracy. The vote in May of last year, held after nearly a decade of stultifying military rule, was expected to be no different.

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