Mitt Romney records anti-Trump robocall for Marco Rubio

Romney's warning is coming to a phone near you in four states voting Tuesday: Michigan, Idaho, Hawaii and Mississippi.

Mitt Romney, the former Republican presidential nominee, talks with people after speaking during the Republican National Committee's winter meeting aboard the USS Midway Museum on Jan. 16, 2015, in San Diego.

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March 7, 2016, 11:29 PM EST / Updated March 7, 2016, 11:34 PM EST

By Peter Alexander, Peter Alexander and Tim Stelloh, Tim Stelloh

In his new role as Donald Trump's chief adversary, former Republican presidential nominee Mitt Romney has turned his alarm-fueled message into a robocall on behalf of struggling GOP contender Sen. Marco Rubio, NBC News has confirmed.