Well the majority of the electorate back in 1960 did not identify with Kennedy's Catholic faith but yet he did win big in parts of the country where Roman Catholics were a small minority. Likewise, in the most recent presidential election many of the electorate voted for Obama even though the vast majority of them did not identify with him racially and ethnically.
There are some on the extreme left, such as the Huffington Post and the Daily Kos, that might make an issue of Romney's religion. But the Democrats would have to be careful of playing that card for it could easily backfire and paint them as religious bigots.
What ironically might Romney in TL or Scenario with evangelical Christian voters and voters in the South is the fact the Democratic presidential candidate is a Jew from the northeast-as VP Lieberman would most likely be the Democratic nominee if Gore had served two terms as POTUS. The two candidates would cancel out each other when it comes to the issue of religion
The Huffington Post and Daily Kos are by no means extreme leftists - they're just partisan liberals and Democrats. RedState is likewise partisanly conservative and Republican, without being anything like the extreme right. Both sides are just well motivated, not politically fringe. Daily Kos split between Clinton Edwards and Obama in 2008, not wvwn between Kucinich, Gravel, and McKinney.
I think the difference between Mormonism in 2008 and Catholicism in 1960 is that people are still only moderately familiar with Mormonism - a national campaign would likely lead to a lot more discovery about the religion and focus on some things that have greatly offended people of other faiths. Most notably against a Jewish candidate, I think the Mormon church's baptism of Holocaust victims would put Romney in an embarrassing spot