Republican presidential candidate Ted Cruz arrives for a Wisconsin primary night rally at the American Serb Banquet Hall in Milwaukee.

Republican Ted Cruz easily won the Wisconsin presidential primary on Tuesday, dealing a blow to front-runner Donald Trump’s hopes of amassing enough delegates for the party’s nomination and boosting chances of a rare contested convention.

Cruz’s double-digit win over Trump was a breakthrough for Republican Party forces battling to block the controversial New York billionaire, and it raised the prospect of a prolonged nomination fight that could last to the July convention in Cleveland, Ohio.

Democratic presidential contender Bernie Sanders also won in Wisconsin, gaining momentum in his fight against front-runner Hillary Clinton and trimming her commanding lead in delegates.

Trump entered the night with 737 convention delegates to Cruz’s 481, leaving him 500 delegates short of the 1,237 needed to become the party’s nominee in the Nov. 8 election.

Cruz said the result in Wisconsin showed the party was beginning to rally behind him, but he acknowledged the growing possibility that the fight could go all the way to the convention.

“Either before Cleveland, or at the convention in Cleveland, together we will win a majority of the delegates and together we will beat Hillary Clinton in November,” Cruz told cheering supporters in Milwaukee.

“We’re winning because we’re uniting the Republican Party.”

Cruz, a conservative US senator from Texas, was aided in Wisconsin by Republican Governor Scott Walker, who dropped his own presidential bid in September, and by a barrage of ads from Super PACS – independent funding groups – backed by party establishment figures worried that Trump will lead Republicans to a broad defeat in November.

Trump’s campaign released a blistering statement saying Cruz had been propped up “by countless millions of dollars of false advertising” from anti-Trump Super PACs.

“Ted Cruz is worse than a puppet – he is a Trojan horse, being used by the party bosses attempting to steal the nomination from Mr. Trump,” the Trump campaign statement said.

The Wisconsin primary capped a difficult week for Trump, who was forced to backtrack after saying women who have abortions should face punishment if the procedure is outlawed, and who voiced support for his campaign manager after he was charged with misdemeanor assault for grabbing a reporter.

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