WASHINGTON: Donald Trump is facing pressure to adopt a more serious tone in his US presidential campaign from Republicans worried that a string of missteps may do him lasting damage in remaining state-by-state contests needed to clinch the nomination.

Those who have marveled at Trump’s rise are cautioning the front-running New York billionaire as weaknesses have emerged in his shoot-from-the-lip approach to campaigning for the party’s nomination to the Nov. 8 election.

Tuesday could be a turning point in Wisconsin where Trump’s leading rival, US Senator Ted Cruz, leads in opinion polls ahead of the state’s primary.

A Cruz win would make it harder for Trump to reach the magic number of 1,237 delegates needed to secure the nomination before the Republican national convention in July.

“If he continues to fumble the ball, he risks everything,” said David Bossie, who as president of the conservative group Citizens United has helped to introduce Trump to grassroots conservative activists. “These types of ham-handed mistakes give his opponents even greater opportunity.”

A 69-year-old businessman and former reality TV show host, Trump has never held public office but hails his mastery of negotiating business deals as the sort of experience a US president needs to be successful at home and abroad.

In recent days Trump has made declarations that sent ripples through a party accustomed to promoting a muscular US foreign policy, suggesting that NATO is obsolete and that Japan and South Korea might need to develop nuclear weapons to ease the US financial commitment to their security.

His statement that women should be punished for getting abortions if the procedure is banned triggered alarm bells among social conservatives who, even though they oppose abortion, do not support punitive measures against women. Trump backtracked hours later.

During a Thursday meeting at the Republican National Committee in Washington with Trump, chairman Reince Priebus addressed any confusion Trump may have had about delegate allocation rules that will govern the July convention, a source familiar with the meeting told Reuters.

Should Trump fail to win enough delegates to secure the nomination outright in the state-by-state contests ending in June, party delegates will select a nominee at the convention in a complex process of sequential votes.

Online predictions market PredictIt said on Friday that the probability Trump will win his party’s nomination had dropped sharply in the past week while the likelihood of a contested convention to choose another candidate had risen.