On a day that Republican Mitt Romney describes as being a “big day” for him in the Presidential race, President Barack Obama’s re-election campaign is releasing a television advertisement accusing Romney of failing to stand up to “the voices of extremism” in his party.

The ad was released Tuesday as Romney was poised to clinch the Republican presidential nomination in the Texas primary.

It takes the former Massachusetts governor to task for failing to speak out against real estate mogul Donald Trump, a supporter who has consistently charged that Obama is not a U.S. citizen.

The commercial opens by showing 2008 nominee John McCain brushing aside a woman who raised the citizenship issue at a town hall-style meeting, and asks, “Why won’t Mitt Romney do the same?”

A Romney aide is shown telling a TV interviewer that “a candidate can’t be responsible for everything a supporter has said.”

Romney is only 58 shy of the 1,144 needed to become the nominee. He  will be in Las Vegas on Tuesday attending a fundraiser hosted by Trump.

(Copyright 2012 by The Associated Press. All Rights Reserved.)