Donald Trump, under pressure to show a more presidential image, elevated a top adviser on Thursday and said he planned to hire additional staff to prepare for the possibility of a long fight for the Republican nomination.

The developments came as Trump tries to rebound from a loss in Wisconsin on Tuesday to U.S. Senator Ted Cruz of Texas, a defeat that punctured the billionaire businessman's aura of inevitability and made it more likely the nominee will be picked in a floor fight at the Republican National Convention in July.

The Republican front-runner's stumble in Wisconsin exposed a variety of weaknesses. Trump alienated many female Republican voters by saying that women who have an abortion if the procedure were banned should be punished, a position he later backtracked on.

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He also appeared to have been outmaneuvered in the scramble for delegates in Louisiana. Although Trump beat Cruz in that state on March 5, Cruz may end up with more delegates from Louisiana than Trump.

Trump announced that he has assigned all functions related to the nomination process to veteran political operative Paul Manafort, who was hired to manage the process of corralling delegates who will pick the nominee.

"The nomination process has reached a point that requires someone familiar with the complexities involved in the final stages," Trump said.

Trump also canceled plans for a campaign swing through California in coming days in order to concentrate on New York, which has suddenly emerged as a must-win state for him on April 19. He has a big lead in his home state.

A Trump campaign statement said that he will add to his team as the Republican National Convention nears and that he would announce new hires in the weeks ahead.

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The expansion of Trump's campaign means his team will grow beyond the close-knit group of advisers who have been at his side since he jumped into the presidential race last June. Trump said he plans to open a Washington office next week.

While there has been speculation that Trump might make Manafort his campaign manager, the candidate's statement made clear that current campaign manager Corey Lewandowski and deputy manager Michael Glassner would remain in their positions.

Lewandowski has been charged with simple battery for grabbing a reporter who tried to ask Trump a question in Florida a month ago. Trump, famously loyal to people he trusts, has rejected calls that he fire Lewandowski.