Key evangelical leaders met a month or so ago and decided that Santorum was their guy. Many in the group supported Gingrich. So, since those two are Catholic and the group was non-Catholic, then I guess having a protestant isn’t at all necessary.
And if you mean “mainline protestant”, well, there aren’t enough of them in America to really worry what they think. Besides those denominations are all bought and paid for by the democratic national committee.
I just don't think the idea of electing a Catholic is as "scary" now as it was in decades past. Fifty years ago it was possible for a Southern Baptist (especially if he didn't serve in the armed forces) to go his entire life and never knowingly meet a Catholic, today that isn't the case. A great many southern Evangelicals have elected Catholics as governors and members of Congress, the old idea that the "pope will be running things" seems to have been gone away.
That being said, I do think that many Evangelicals AND Catholics would have a huge problem with a Mormon nominee and the media would like nothing more than to run all sorts of TV specials on the history of Mormonism.