The National Review has published a good dissection of the GOP’s problems and the reasons for Romney’s loss. Ramesh Ponnuru focuses on the GOP’s overall un-likability:
Romney was not a drag on the Republican party. The Republican party was a drag on him. Aaron Blake pointed out in the Washington Post that Romney ran ahead of most of the Republican Senate candidates: He did better than Connie Mack in Florida, George Allen in Virginia, Tommy Thompson in Wisconsin, Denny Rehberg in Montana, Jeff Flake in Arizona, Pete Hoekstra in Michigan, Deb Fischer in Nebraska, Rick Berg in North Dakota, Josh Mandel in Ohio, and of course Todd Akin in Missouri and Richard Mourdock in Indiana. In some cases Romney did a lot better. (He also did slightly better than Ted Cruz in Texas, a race Blake for some reason ignored.)
None of those candidates were as rich as Romney, and almost all of them had more consistently conservative records than he did. It didn’t help them win more votes. The only Republican Senate candidates who ran significantly ahead of Romney were people running well to his left in blue states, and they lost too.
He then runs down the recent history of national elections:
The 2012 Senate races were more like the ones in 2006 and 2008: wipeouts for Republicans of every description — veterans and newcomers, conservative purists and relative moderates alike.
All these candidates lost not because of the idiosyncrasies of this or that candidate or the flaws of this or that faction of the Republican party. They lost not because of the particular vices of the Tea Party, or of social conservatives, or of the party establishment. The most logical explanation for the pattern is that something common to all Republicans brought them down, and the simplest explanation is that their party is weak — and has been for a long time. Consider the evidence: Republicans have lost the popular vote in five of the last six presidential elections. Since the Senate reached its current size, Democrats have had more than 55 seats 13 times; Republicans, never…
…Republican weakness emerges even more clearly when we look at a longer timescale. From 1896 through 1930, Republicans were the dominant party, holding the White House and Congress most of the time and losing the presidency only when they split, as in 1912 and 1916. The Great Depression made the Democrats into the dominant party until 1968. Only one Republican won the presidency during that period, and under highly unusual circumstances: He had won World War II, the Democrats had held the presidency for five consecutive terms, and the country was beset by inflation, corruption, and an unpopular war in Korea.
The Democrats lost majority status in 1968 — they would lose five of six presidential elections from that year through 1988, and win one by a hair — but Republicans did not gain it. They never held the House and rarely held the Senate during that streak of presidential wins. Why didn’t Republicans become the dominant party then? It wasn’t because of foreign policy: That boosted them during the second half of the Cold War, when the Democrats became the relatively dovish party. That’s a big reason Republicans did better at the presidential than at the congressional level. It wasn’t because of social issues: The hippies and McGovernites helped make Republicans the party of middle-class values.
What they did not do is make the Republicans the party of middle-class economic interests.