The Christian News Wire reports that a social conservative group has threatened to pull support from John McCain if he picks Mitt Romney as his vice presidential running mate.

More than 20 leading social conservatives signed an open letter to Senator John McCain expressing their displeasure over the prospect of an "M&M" ticket being pushed by Karl Rove, Sean Hannity and others in the economic wing of the Republican Party.

UPDATE: A PAC that calls itself Government is Not God plans to run the ad in cities where McCain is campaigning. The full-page ad reads:

If Governor Romney is on your ticket, many social conservative voters will consider their values repudiated by the Republican Party and will either stay away from the polls this November or only vote down ticket. For the sake of your election, the health of your party, and the future of America you must not allow the obvious electoral consequences of that to occur.

Romney has started downplaying being McCain's running mate, telling CNN McCain has not told him he is on the 20-name short list for running mates:"I don't think I'm very likely."

And earlier on CNN, McCain ominously called Romney "a fine man, who has earned himself a large place in our Republican Party."

Paul Weyrich, who endorsed Romney during the primaries, is a part of the Government is Not God PAC, but says he now regrets it.