At a rally in Tuscon, AZ yesterday Trump campaign manager Corey Lewandowski was involved in an altercation with a protester. As you can see in the above video, Lewandowski appears to grab a protester by the collar as another man more forcefully holds him, to which the man reacts.

The Trump campaign denies any wrongdoing by Lewandowski. Campaign aide Hope Hicks told the Guardian “The video clearly shows the protestor reacting to the man who pulled him, not to Mr Lewandowsk. Mr Trump does not condone violence at his rallies, which are private events paid for by the campaign.”

Lewandowski was involved in an assault on Breitbart reporter Michelle Fields on March 8th. Charges have been filed against Lewandowski for that incident. The Trump campaign has been marked in recent weeks by a startling uptick in violence. Police and security have clashed with supporters and protesters several times in the past week. At the same rally in Tuscon yesterday a protester was more seriously assaulted by a supporter, as the crowd roared:

Sources:
Video: https://www.youtube.com/watch?v=mXmsXNKh1pM
Video: https://www.youtube.com/watch/t_WScwLKfR0
http://www.cnn.com/2016/03/19/politics/trump-campaign-manager-protester/
http://www.theguardian.com/us-news/2016/mar/19/trump-campaign-manager-lewandowski-arizona-rally