09/10/2010 02:55 pm ET Updated Jun 18, 2012

Registered vs. Likely Voters: How Large a Gap?

According to several recent national polls, Democrats may be headed toward their worst showing in a congressional election since World War II. A new NBC/Wall Street Journal Poll has Republicans leading Democrats on the generic House ballot by 9 points among likely voters while a new Washington Post/ABC News Poll has Republicans with an astonishing 13 point lead. The most recent Rasmussen weekly tracking poll has Republicans with a 12 point lead among likely voters.

If these polls prove to be accurate, Republicans could achieve their biggest popular vote margin since the 1920s. In 1946, Republicans won the national popular vote for the House of Representatives by a margin of about 9 points and that was their biggest win in the past 64 years. The Republicans' second biggest popular vote margin was 7 points in 1994.

What would such a popular vote margin mean in terms of seats? In 1946, Republicans won 246 seats in the House--a gain of 56 seats over their previous total of 190. A 12 or 13 point Republican margin would likely produce close to 260 Republican seats--a gain of about 80 seats over their current total of 179. That would be the biggest seat swing in a House election since 1932 when Republicans lost 101 seats. It would dwarf the 1994 shift when Democrats lost 52 seats, their worst showing since 1946.

It is very likely that Republicans will make substantial gains in this year's midterm election. Democrats are defending many seats in Republican-leaning districts that they picked up in 2006 and 2008, Americans are very anxious about the condition of the economy, and President Obama's approval rating has fallen into the low-to-mid 40s in recent weeks. My own forecasting model now has Republicans gaining between 40 and 50 seats in the House. But how realistic are polls that show Republicans winning the national popular vote by a double digit margin-- enough to produce record-setting Democratic losses?

There is one reason to be skeptical about some of these recent poll results--they reflect an enormous gap between the preferences of registered and likely voters. Rasmussen does not release generic ballot results for registered voters, nor do they provide any information about how they identify likely voters. But the recent NBC/Wall Street Journal Poll reported a tie on the generic ballot among registered voters. Likewise, the new Washington Post/ABC News Poll reported only a 2 point Republican advantage among registered voters.

It is not surprising that Republicans would be doing better among likely voters than among all registered voters, especially in a low turnout midterm election. Republicans generally turn out in larger numbers than Democrats because of their social characteristics and this year Republicans appear to be especially motivated to get to the polls to punish President Obama and congressional Democrats. But a double-digit gap between the preferences of registered and likely voters is unusually large.

According to data compiled by the Gallup Poll, in 13 midterm elections between 1950 and 2006 for which relevant data were available, the average gap between the preferences of registered and likely voters was 5 points. Only once, in 2002, did the gap reach double digits. In that year Democrats had a 5 point lead among registered voters but Republicans led by 6 points among likely voters. However, the gap in party preference between registered and likely voters did reach 9 points in 1962 and 8 points in both 1974 and 1982 and in every one of these years, the preferences of Gallup's likely voters were closer to the actual election margin than the preferences of registered voters. In fact, across all 13 midterm elections, the Democratic margin among likely voters differed from the actual Democratic margin in the national popular vote by an average of only 2.1 percentage points while the Democratic margin among registered voters differed from the actual Democratic margin by an average of 6.5 percentage points.

These results appear to support two conclusions. First, while a double-digit gap between the preferences of registered and likely voters is unusual, based on the history of Gallup's generic ballot polling, it is not unprecedented. Second, result of the final Gallup generic ballot among likely voters has been a very good predictor of the national popular vote for the House of Representatives. If that poll finds Republicans with a double-digit margin, Democratic losses in November could be substantially greater than those the party suffered in 1994.