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 court also barred from politics for a decade 11 party members and executives, including Pita Limjaroenrat, its former leader and prime ministerial candidate.

But stamping out the party could reignite the discontent that fueled its rise in the first place.

Breaking up Move Forward would not end the progressive movement’s “journey to change Thailand,” Mr. Pita said in Parliament in April. On the contrary, he said, it could “make us reach the victory line” even faster.

In an interview around that time, Mr. Pita said the old guard’s intentions were clear. “This is trying to get rid of opposition,” he said. “And without opposition, there’s no democracy.”

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.