Donald Trump campaign manager Corey Lewandowski has been charged with assaulting a journalist at a campaign event.
Mr Lewandowski is charged with simple battery over his encounter with former Breitbart reporter Michelle Fields.
Police allege that on 8 March after a news conference in Florida, he grabbed her arm when she tried to ask Mr Trump a question, leaving a bruise.
Mr Lewandowski plans to plead not guilty, the Trump campaign said.
Police in Jupiter, Florida, where he was arrested, have released new footage of the alleged assault.
The security video appears to show him trying to pull Ms Fields out of the way as she walks alongside Mr Trump and tries to speak to him.
“Mr Lewandowksi is absolutely innocent of this charge,” the Trump statement said. “He is completely confident that he will be exonerated.”
Mr Trump himself has defended Mr Lewandowski on Twitter, saying the police footage shows there was “nothing there”.
Bad day for Trump – Anthony Zurcher, BBC News, Washington
If Donald Trump is going to win the Republican nomination without a convention fight, he will have to endure yet another string of political blows that would stagger even the strongest campaign.
A week before a key primary in Wisconsin, Scott Walker – the state’s governor – gave a full-throated endorsement of rival Ted Cruz. Now Mr Lewandowski has been charged over his manhandling of a female reporter during a rally. This comes after a particularly tawdry few days in the Republican presidential battle as Mr Trump and Mr Cruz engaged in a war of words over their spouses.
The New York property developer has been able to maintain his front-runner status for months despite countless controversies and conflagrations. This time may be different, however.
Recent events are likely to cause further damage to Mr Trump’s already poor standing with female voters. The Republican presidential pack has been whittled to just three candidates, with the not-Trump vote consolidating behind Mr Cruz.
Mr Trump’s path to securing the Republican nomination is a narrow one, so the slightest stumble could deny him the prize outright. Then it’s on to the convention, where party loyalists and a growing cadre of Cruz-friendly activists hold sway.
The charge comes during a difficult time for Mr Trump, just ahead of next week’s Wisconsin primary where he is neck-and-neck with Senator Ted Cruz.
Earlier on Tuesday, Wisconsin Governor Scott Walker endorsed Mr Cruz for president.
Mr Trump will participate in a CNN town hall event on Tuesday night when all three Republican contenders will face questions.
He is currently well ahead in the Republican race with 739 delegates to Cruz’s 465.
Ohio Governor John Kasich is some way behind with 143, with the 1,237 needed to win the nomination probably out of his reach.