A party that attracts new support from a segment of the electorate tends to repel part of its old coalition. As the 1990s began, political pundits were opining that Republicans had a lock on the Electoral College—just before Bill Clinton, with assistance from Ross Perot, picked the lock and ripped open the door. Democrats won the popular vote in four of the five next presidential elections.
Republicans similarly embarrassed the pundits who said two decades ago that Democrats had a lock on the House of Representatives. Republicans won the most popular votes and most seats in seven of the nine congressional elections beginning in 1994.
As a result, the Republican core going into the 2012 election is no longer northern Protestants but white, married Christians. If you compare John McCain's 2008 percentages with the senior Bush's in 1988, you find Republicans suffering double-digit losses in states dominated by giant, culturally non-Southern metropolitan areas—New England, New York and down the I-95 corridor through New Jersey, Delaware, Maryland, Virginia and then down to Florida.
Republicans found support similarly sparse in Illinois, Michigan and (though the loss here seems temporary) Indiana, as well as in California, Nevada and Barack Obama's native Hawaii. Latino immigrants have added Democratic votes in most of these places, and out-migrating, native-born Americans have subtracted Republican votes.
But when you look at this map you also see areas where Mr. McCain ran even with or better than George H.W. Bush, notably the (Andrew) Jacksonian belt settled by Scots-Irish in the 18th and 19th centuries, running southwest from West Virginia through Kentucky, Tennessee, Alabama, Arkansas, Louisiana and Oklahoma. Western Pennsylvania and rural Texas, which remained solidly Democratic in the 1980s, have become solidly Republican.