Donald Trump has vowed to stand by his campaign manager Corey Lewandowski after he was charged with simple battery following an altercation with a female reporter.
Trump spoke on his plane shortly after landing in Wisconsin for a rally ahead of the state's April 5 primary.
Police in Jupiter, Florida, issued Lewandowski with a notice to appear before a judge on May 4 for the misdemeanour charge. A surveillance video released by the police appears to show Lewandowski grabbing a reporter for Breitbart News as she tried to ask Trump a question during a March 8 campaign event.
The charge carries up to a year in jail.
The Trump campaign said Lewandowski "is absolutely innocent of this charge".
"He will enter a plea of not guilty and looks forward to his day in court," said the statement. "He is completely confident that he will be exonerated."
Trump himself called Lewandowski "a very decent man" on Twitter: "Look at the tapes – nothing there!"
"Lewandowski grabbed Fields' left arm with his right hand causing her to turn and step back," reads the report. Fields showed police her left forearm which "appeared to show a grabbing-type injury," according to the investigating officer.
In a second piece of bad news for Trump, Wisconsin governor Scott Walker endorsed his rival Ted Cruz in the Republican race for the party's nomination.
Walker said his endorsement was not part of mainstream Republicans' drive to block front-running Trump but support for "a strong new leader".
The Wisconsin governor, who was briefly in the race for the Republican nomination, announced his support for Cruz and said he is "in the best position by far to both win the nomination of the Republican Party and then go on to defeat Hillary Clinton in the fall".
Trump planned to make his first campaign appearance in Wisconsin before joining Ohio governor John Kasich and Cruz for a CNN interview.
It is unclear how much Walker's endorsement will help Cruz. The governor's approval rating has not cracked 40 per cent in more than a year.
Wisconsin has 42 delegates, with 18 going to the state-wide winner and 24 divided among the winners in each of the state's eight congressional districts. Trump heads into Wisconsin with 739 delegates to Cruz's 465. Kasich lags behind with 143. It takes 1,237 to assure the nomination.