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No evidence for Mellman's "magic" 50% approval

Topics: Barack Obama , midterm , presidential approval

Last week, I criticized Matt Bai's claim that it was an "ominous sign" for Democrats that President Obama's approval rating is under 50%.

Writing in The Hill, top Democratic pollster Mark Mellman goes even further, calling 50% approval a "magic number" for midterm elections:

[P]erhaps there are some magic numbers after all. 

Take the effect of presidential approval on midterms. We graph the relationship between a president's approval rating and his party's gains and losses in midterm elections, thinking of the result as a smooth relationship. The lower the president's approval rating, the more seats his party loses.

But the pattern is not really so linear after all. There is a sharp discontinuity at 50 percent. Presidents whose approval rating is at 50 percent or above have lost, on average, just 11 seats in the House, while presidents under the 50 percent mark have lost an average of 33 seats.

Averages can obscure as much as they reveal, so pick apart the numbers. No president with an approval rating under 50 percent has lost fewer than 15 seats. The next smallest number is 26. Even a president just below 50 percent can lose a lot. When Democrats were punished with the loss of 52 House seats in 1994, President Clinton's approval rating rested just under the 50 percent threshold, at 48...

So where does President Obama stand? Last week Gallup put his approval at 45 percent, this week at 49. The Pollster.com weighted average of all polls says 46 percent. In short, for now, the president is hovering just below what may prove to be a magic number for Democrats in 2010.

Unlike Bai, Mellman brings historical evidence to the table, so let's consider his argument. He acknowledges the seemingly linear relationship between presidential approval and changes in House seats for the president's party in midterm elections. However, he argues that there is a "sharp discontinuity at 50 percent" in which "[p]residents whose approval rating is at 50 percent or above have lost, on average, just 11 seats in the House, while presidents under the 50 percent mark have lost an average of 33 seats."

First, let's replicate Mellman's numbers. (He appears to be excluding the replacement presidents -- Truman in 1946 and Ford in 1974 -- so I do the same here.) Using a cutpoint of 50 percent, I find that presidents with greater than 50 percent approval in the most recent Gallup poll before Election Day lose an average of ten seats and those below 50 percent lose an average of 33.5 seats (these slight discrepancies are likely the result of how different sources calculate seat change).

The problem is that the choice of 50 percent is arbitrary. For instance, presidents with approval ratings above 45 percent lost an average of 15 seats, while those below 45 percent lost an average of 33 seats -- results that aren't that different from Mellman's original numbers. Going the other direction, any cutpoint from 51 percent to 56 percent will yield the same results as 50 percent because there are no presidents who had approval ratings in that range in the data.

If you prefer graphical evidence, here is the data with a standard linear fit:

Linearmidterm2

If we instead use a more flexible local polynomial fit to allow for nonlinearity, the predicted values show an inflection point around 50 percent approval, but the 95% confidence intervals reveal a great deal of uncertainty in that estimate -- hardly enough to justify a claim of a "sharp discontinuity":

Lpolymidterm2

With so few data points, it's very difficult to demonstrate a non-linear relationship. Absent further evidence, we can't be confident that a discontinuity exists at 50 percent.

It's also hard to believe the claim of a discontinuity in the context of the upcoming midterm elections. Given the state of the economy and the generic ballot, it's clear that Democrats are likely to lose a substantial number of seats regardless of whether Obama's approval rating is 49 percent or 51 percent on Election Day. Does Mellman believe otherwise?

Update 6/24 4:16 PM: To illustrate the point a bit further, I created a simple simulation of a linear relationship between approval and seat change that produces data approximately similar to what we observe above:

Mellman-sim2

Over 1000 iterations of the simulation, the average outcome for presidents below 50 percent approval was a 12 seat loss while the average outcome above 50 percent approval was a 29 seat loss. In other words, it is very easy for a linear relationship to produce the sort of outcomes that Mellman describes.

[Cross-posted to brendan-nyhan.com]

 

Comments
hoosier_gary:

If presidential approval was the one and only factor in mid-term congressional elections, all this time spent on the relationship between presidential approval and seat loss would have some more merit.

Presidential approval is only one factor involved in mid-terms. You also have to factor in that congress has the lowest approval in history (20%).

You also have to factor in that congress passed some massive bills on strictly party-line votes that a majority of voters strongly oppose.

You have to factor in those 29 congressman who were elected in districts that voted for McCain.

You also have to factor in 10% unemployment - especially when Obama and congress swore that it wouldn't go above 8% if they spent a trillion dollars.

You have to factor in that 63% of the country thinks we are headed in the wrong direction.

You have to factor in the historically high number of democrats in congress. There is no ambiguity about who controls congress and who is making these very unpopular votes.

You are also talking about a 49% or 51% approval for Obama in November. His approval fell below 50% in August of last year and has been dropping ever since. His composite average is at 45.8% What makes you think that is going to jump in the next 4 months? You should be looking at charts based on his approval at 46%.

All of those together start to paint a picture of a minimum seat loss of 50.

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Personally, I don't think any of these analyses have any merit. We live in very uncertain times and anything can happen. I can construct a very realistic model that could show Democrats losing 20-30 seats in the House and gaining 2 seats in the Senate, simply by charting individual races.

The typical American scenario that has the party of the President losing seats in the following off-year election has, as its basis, the rationale that the President's party picked up extra seats during his presidential election campaign by virtue of his coattails and people voting not only for the president but also for his party. Then, two years later, when the president wasn't on the ballot to have the coattail effect, those seats reverted to the norm.

However, the main seat gain won by the Democrats occurred in 2006, not 2008 and there was very little coattail effect for Obama. The GOP got wiped out in the House in the northeast and New England over three elections beginning in 2004 due to their sharp veer to the right.

They may win back a handful of those seats, but not many, since the public in those states still despises GOP politics, probably even worse today due to the influence of the Palins, the Jan Brewers and the Joe Bartons of the world, to name just a few.

The first Tea Party occurred almost two and have centuries ago in Boston. Even Scott Brown rejects that political philosophy today.

the mountain west has some Democratic Congressional gains in the last two elections. Those seats are probably at risk. There are very few Democrats in the south, except in minority/majority districts, so where would they lose seats there?

In the industrial midwest there will probably be a few Democratic seats turn over, but not that many. 5-10 tops?

I don't see a lot of vulnerability for Democratic seats on the West Coast, since the Democrats holding seats there hold them in typically very safe districts.

So where will this massive seat pick-up occur? Can somebody tell me?

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Rockym92:

The seat pickups will occur all across the country. Only counting red and purple districts, there are 54 in the south and midwest alone. Another 17 in the northeast and 14 in the west. That is not counting districts that narrowly went for the dem candidate in 2004 and 2008 of which there are a handful more in possible danger.

90+ potential R pickups opposed to only 5 or 6 that the dems have any shot of taking. As of right now the dems are looking at a 30+ seat loss. If things dont improve or somehow actually get worse in this country the wave could be massive and start taking out dems that were previously thought of as safe. On the flip side, If there is a noticeable improvement over the next few months, the dems could limit their losses to under 20. That would be a major achievement, but is very unlikely as of right now.

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William Ockham:

One technical note: if you exclude Truman 46 and Ford 74, you should excude Johnson 62.

Of couse, that doesn't really matter because this whole thing is silly. The people expecting massive Dem losses will be sorely disappointed. The Dems will end up with more seats than they had in 2006.

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