The ad is sponsored by God is Not Government PAC, headed by William Murray (the fundamentalist son of the late atheist leader Madaline Murray O'Hair).
Murray writes:  

I had the unfortunate experience of hearing Senator John McCain speak just after he secured enough delegates to force Governor Mike Huckabee to withdraw from the race. It was just pathetic. He spoke for about 10 minutes at a lunch I was attending and then allowed questions. His theme was from the era of the 1980's or 1990's, addressing none of the current concerns of Americans, other than Iraq. He did not touch on the price of oil even though it hit a record high that day. When asked about his faith he gave the testimony of another man instead of his own, and this left everyone in the room scratching their heads.

As for his vice presidential selection ideas, he started off his speech by telling us what a great guy Mitt Romney is, with a "bright" future in the Republican Party. Meanwhile Karl Rove is pushing what he calls an "M & M ticket" for McCain and Mitt. I can think of few choices that would be more of a disaster. If John McCain wants to insult the hundreds of thousands of people who voted for Mike Huckabee, the best way to do it would be to pick Mitt Romney for Vice President. A marginal Christian with a Mormon sidekick is not going to excite social conservatives, particularly evangelicals.

In light of other recent anti-McCain rumblings on the religious right, it is looking like a rough ride for the GOP ticket.

CNN has also now done a report on this.