Donald Trump is fighting to put a difficult week behind him and finish strong on Tuesday in Wisconsin, a state whose primary contest may prove to be a turning point in the race for the Republican presidential nomination.

The Republican front-runner is at risk of losing the Midwestern state to Senator Ted Cruz of Texas, an outcome that would dent the New York billionaire's aura of inevitability and make it harder for him to win the 1,237 delegates needed for the party's nomination for the Nov. 8 election.

On the Democratic side, U.S. Senator Bernie Sanders of Vermont is trying to hold on to a slender lead over front-runner Hillary Clinton in the opinion polls in Wisconsin and eke out another victory over the former secretary of state.

Trailing Cruz in the polls in Wisconsin, Trump spent a good deal of a lengthy speech on Sunday night to supporters in West Allis, Wisconsin, attacking Cruz as a liar and a "dirty rotten cheater" who is weak on immigration and would cut Social Security benefits.

"Wisconsin is going to be such a big surprise on Tuesday. We are doing so well," Trump said.

A loss would add to Trump's woes after his campaign was rocked last week by the fallout from his suggestion, which he later dialed back, that women be punished for getting abortions if the procedure is banned.

He also drew fire for comments that he would not rule out using nuclear weapons in Europe and that Japan and South Korea might need nuclear weapons to ease the U.S. financial commitment to their security.

"Was this my best week? I guess not," Trump told "Fox News Sunday" in an interview conducted last Friday. But he added: "I think I'm doing OK."

Cruz, speaking to supporters on Sunday in Green Bay, Wisconsin, was eager to take advantage of whatever opening Trump had provided him.

More and more Republicans are recognizing, said Cruz, that "nominating Donald Trump would be a train wreck - and that's not fair to train wrecks - (and) that nominating Donald Trump elects Hillary Clinton. Hillary wins and she wins by double digits."

Cruz faces difficulty in winning the delegates needed to win the nomination, given that the next round of states to vote, such as New York on April 19, are friendly territory for Trump.

Both Cruz and the third Republican left in the race, Ohio Governor John Kasich, are trying to deny Trump enough delegates so that the nomination fight is decided at the Republican National Convention in July.

Trump complained that Kasich was an irritant for remaining in the race and gobbling up some of the delegates Trump needs.

"The problem is he’s in the way of me, not Cruz," Trump said. "He hurts me more than he hurts Cruz."

Kasich vowed to stay in the race.

"That's not how our republic works, Donald," Kasich tweeted in response. "We'll keep fighting until someone reaches a majority of delegates."

(Reporting by Steve Holland; Editing by Peter Cooney)