|The House result is usually a mix of two factors, environment and the seats' natural lean. Environment is a well known phenomenon. It can be coat tails for a Presidential, a negative reaction to an unpopular President, or in an unusual circumstance rallying around a President. In many cases the environment is fairly neutral.
Most congressional districts will go to a certain party in a normal environment. If one party controls districts which lean to the other party as a result of a wave, a neutral environment will return most of those to that party. Thus, 2010 was going to be a GOP year regardless, since the party was nearly 50 seats below the seats' natural party lean. Republicans would've made big gains, just not that big, even without the wind at their backs.
Here are the second term mid-terms. If a President has a bad second mid-term he'll usually have had a first mid-term without real losses. Presidents just don't have two horrible mid-terms. There are probably a lot of reasons for this.
1950 - I'm sure this looks like a bad mid-term for Harry Truman, but it really wasn't. His enormous win in 1948 resulted in Democrats getting 263 House seats, about 30 more than they'd get in a normal year. Without Truman's momentum Democrats lost 28 seats, to put them at 235, about where they'd expect to be in this era. Truman had a very bad mid-term in 1946 and this one wasn't.
1958 - While Republicans had lost the House in 1954, this was just simply a return to normal. The Democrats had 232 seats in 1954 and 234 in 1956. Unlike 1954, this was a horrible environment for Ike and the Republicans. They got thrashed at the polls and lost 48 seats. The Democratic conference was bloated with seats they couldn't hold as a result. Thus, they lost seats in 1960 even though Kennedy won the Presidency.
1966 - Johnson's 1964 landslide gave Democrats 295 House seats, a conference they couldn't hold. The era average was 256 and Democrats lost a little more than that, dropping to 247. Kennedy hadn't lost seats in the first mid-term.
1974 - Republicans were about 13 over their era average, so losses were likely. Add in Watergate and Democrats picked 49 seats. Nixon's first mid-term had been mild.
1986 - After the 1984 election Republicans stood only 3 seats above the era average. Democrats gained 5 seats. Reagan had had a tough first mid-term, but not a second.
1998 - The Democratic performance here has been attributed to the impeachment. That certainly helped the Democrats, but both sides entered the year at their era averages. So there wasn't the opportunity districts of 1994.
2006 - Bush had a mild first mid-term, but not the second. The environment was terrible for Republicans and the Democrats gained enough seats to put them 26 over the era average. That makes this mid-term similar to 1958 when Democrats were 27 above their era average.
2014 - Most of the 2012 Democratic pick-ups were Democratic leaning seats. They won a decent share of marginal seats but not many. In 2010 Republicans could identify dozens of R+ seats for pick-up. Democrats probably have a handful now. So the obvious pick-ups aren't there this time, the way they were in 1950 and 1966. It's possible that the election will be like 1958, 1974, and 2006 but those were all in Presidencies that didn't have heavy first mid-term losses.
Thus, I don't see big gains for the GOP in 2014. That said, it won't be a good year for Democrats. There have been 17 mid-terms since 1944 and in 16 of them the President's party has won a lower percentage of the House vote than they did in the Presidential election. This occurred regardless of whether the President's party controlled the House or what vote share the party got in that year. That one exception was 2002, when President Bush's approval rating was sky high due to 9/11. That seems unlikely to happen again.
All the gloom and doomers, especially those saying all our California seats are lost, should consider this when making those pronouncements. The Democrats won a lot of close races here and it'll be hard to hold them in a year that's very likely to be more Republican.
I think Democrats will get 48%-49% of the vote, which should result in losses. I'm thinking the GOP will pick up 8-15 seats.