Ron Paul calls out opponents of the Park 51 project, saying, "This is all about hate and Islamaphobia."
In my opinion it has come from the neo-conservatives who demand continual war in the Middle East and Central Asia and are compelled to constantly justify it.
They never miss a chance to use hatred toward Muslims to rally support for the ill conceived preventative wars. A select quote from soldiers from in Afghanistan and Iraq expressing concern over the mosque is pure propaganda and an affront to their bravery and sacrifice.
What's interesting is that the people justifying their opposition to Park 51 by arguing that the U.S. should be fighting a wider war against "Islamism" instead of just terrorist groups also support indefinite continuation of the wars in Iraq and Afghanistan. Yet both of those conflicts involve using American blood and treasure to prop up Islamist governments. If they really actually believed the U.S. were at war with all forms of religiously motivated political advocacy involving Muslims, they would be radical isolationists, demanding that Americans stop paying and dying in the name of Islamism. Instead they've concocted this conspiratorial narrative about America being on the verge of Islamist subversion while demanding open-ended military commitments that involve propping up American-backed Islamist governments.
Islamism as a blanket term is almost useless, because there are benign forms of Islamism and violent, repressive forms of Islamism. But it's no more possible to prevent Islamist groups from participating in the political process in countries with lots of Muslims than it would be to prevent religiously motivated Christians from organizing for political purposes in the United States.
That's why people like Andy McCarthy like to blur the line between terrorism and any kind of religious political advocacy performed by Muslims--because they believe allowing such participation in the American political process could lead to the imposition of Taliban-style Sharia law in the United States. Considering the relative lack of success conservative American Christians have had in imposing their belief system on the American legal system despite a rather large Christian majority, that scenario seems pretty unlikely, to say the least.