Who is Corey Lewandowski, Donald Trump's campaign manager accused of grabbing a protestor?

Republican candidate Donald Trump was quick to defend his campaign manager, Corey Lewandowski after he was involved in what appeared to be a physical altercation at the a rally in Arizona.

Violence and aggression are fast becoming a hallmark of Trump rallies, and in the latest example of emotions over the outspoken businessman boiling over, his embattled campaign manager Lewandowski seemingly grabbed a protester by the throat.

Earlier in March, Lewandowski allegedly manhandled Breitbart News reporter Michelle Fields while she attempted to ask Trump a question. The Republican frontrunner has faced accusations that he himself has encouraged violence at his events saying that he would "pay the legal fees" for anyone who punched a protester.

After footage emerged of the incident, Trump was quick to defend a key member of staff and denied that an assault took place. He told ABC's This Week: "I give him credit for having spirit. He wanted them to take down those horrible, profanity-laced signs."

Who is Corey Lewandowski, the man behind the Trump's controversial campaign?

The 42-year-old has been advising Trump since January 2015, steering the Republican Party candidate to the brink of the candidacy.

Lewandowski studied at the University of Massachusetts and in 1994, while still an undergraduate college student, he ran for the 16th Middlesex district seat of the Massachusetts House of Representatives, which he lost to Thomas Golden, a Democrat.

He earned an MA at American University and after completing this degree in 1997, he moved to Washington. Lewandowski interned for Democrat Massachusetts State Senator Steven Panagiotakos. He then worked with the Republican National Committee in 2001.

In 2001, Lewandowski got his first gig as a campaign manager, working for Republican US Senator Robert Smith of New Hampshire. The bid was due to fail, though, as John Sununu, the son of the state's former governor won the seat.

From 2003 to 2004, Lewandowski served as the executive director of the New England Seafood Producers Association. Then he became a lobbyist with Schwartz MSL, a Boston-based public affairs firm that represents the technology, healthcare, and green energy sectors in Washington.

He also served with the Koch brothers' Americans for Prosperity and in 2012, backing computer-repair mogul Josh Youssef. Youssef lost in the general election and is now a county chairman for Trump in New Hampshire.

According to Politico, Trump first met Lewandowski as the AFP and Citizens United held a "Freedom Summit" in New Hampshire. They kept in touch over the next few months before Trump hired him in January 2015.