Do midterms naturally favor Republicans or the party out of the presidency?

[Virginiá]

I think there are a number of factors in this:

1. Voting patterns of the current generation of older people (who vote more reliably in midterms): As the Republican-leaning silent generation ages out of the electorate, older boomers (people who turned 18 during Kennedy/Johnson/Nixon admins.), who lean more Democratic, will fill the 60+ age bracket, will probably skew things a bit more Democratic than it is now, but probably not enough to equally counter the influx of Ford/Carter/Reagan/Bush boomers/genXers who lean more strongly Republican. So I think from 2016 to 2022 should be a little better for Democrats than it is now, but after that it will be somewhat more favorable for Republicans again.

2. Coalition/turnout: Who your voters are and how often they vote is important. The Republican party is comprised of older white voters who are more evenly distributed and have good turnout rates, which makes them more influential. Income level also helps them, as does the large amount of politically active religious voters. Democrats, in addition to white voters, have quite a lot of young people / minority voters, who both tend to vote less reliably in midterms (sometimes by a lot).

3. Distribution of voters: If we're talking about 'favor' as in who has an advantage in gains rather than total votes - Then geography is important too. Democratic voters tend to cluster in population centers which act as natural vote sinks. This gives Republicans an edge due the distribution of their voters. This is problem for Democrats is compounded by gerrymandering, but I consider that issue more of a cheat than a natural factor.

4. Who controls the White House: Because the out-party treats the president and his party as a punching bag with often successful results, the in-party tends to not do as well in midterms. People (sometimes ignorantly) tend to blame the president and his party for a lot of things that happen, whether or not they deserve the blame, and take it out on them at the polls.

This is one of the most consistent factors, but wouldn't necessarily mean doom for the in-party, as you can see from 1930 - 1994, with the Democratic coalition and other factors giving them solid control of Congress despite events arising that would normally have cost them control

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There may be others I missed, but I think these primarily influence which party has the current midterm advantage

[Virginiá]

Number of midterms where the Democrats did well despite holding the White House: 3 (1934, 1962, and 1998)

1934 - This was part of the Democratic realignment following the Great Depression. Those were unexpected, unique circumstances that essentially took a blowtorch to the GOP. Plus that was over 80 years ago - The coalitions were not the same, and yes, that does make ALL the difference here.

1962 - What do you mean? Democrats lost 4 seats.

1998 - This is widely perceived to be a backlash against the Republican impeachment attempt. Though, Bill Clinton's approval ratings were around 66% through that election and the country was doing well, so those factors definitely helped. Once again, midterm successes are anchored to the hijinks of reality.

Interesting that people still think midterms favor the GOP...

NONE of the years above means anything really to the current GOP coalition's midterm advantage. In fact, their advantage is generally low turnout elections in general. Their voters tend to be better distributed (not packed into hyper-performance districts like Democrats), they consist mainly of white voters (something like 90% of the Republican party's voters are white), with a majority now I believe being older & more likely to vote. Add in churches and religious groups mobilizing voters and other factors and you get a more reliable base in terms of turnout.

The Democratic party's coalition is only 60% white. That 40% non-white voters are packed into certain regions/cities, making their voting power less effective and their turnout rates are pretty low (Asians/Hispanics typically < 49%, African Americans in 2012 were 66%, though) A lot of the Millennial generation hasn't aged enough to vote more regularly, so they also have low turnout rates. They also have somewhat of a ongoing tendency to cluster, if I recall correctly.

The problem is non-white voters also drop off substantially during midterm elections. So an already weakened voting bloc contributes even less in elections that matter. This kind of behavior leads to good presidential elections and terrible midterms. Until Millennials grow up and vote more/replace the older voters, this will probably be the norm for at least another decade.

So yes, midterms do favor the GOP at this time, but it won't necessarily favor them forever

[Virginiá]

So, in other words, is the Republican base more consistent in turning out (even when the mood is against them), and does this give the GOP a boost (or cushion)?

Is there any way we can try to measure that?

I don't know about measuring it, but the Republican coalition (if you want to call it that, considering how racially homogeneous it is) is more reliable in general and it's better dispersed geographically to better affect election results. This is an advantage they always have, regardless of who is in the White House.

People who say it all depends on what party holds the White House at the time are leaving out a huge number of other factors in favor of a simple answer that just isn't that simple.

[Virginiá]

I mean, maybe I'm wrong and every midterm election will be an easy win for the party that doesn't control the White House, but I kinda doubt it. 2018 will be very telling, in all honesty.

I guess it depends on what you mean by an easy win. Historically, the "out party" almost always picks up seats. It's such a consistent pattern that it would be unwise to bet against it. The question is really how many seats. The fact that Democrats need to win the House PV in what is basically a landslide is a testament to how big the GOP advantage is.