If all adults in America voted, Democrats would be the dominant political party and might not face any net loss of seats in November. The problem for Democrats is that only a portion of adults registers to vote, and only about half of those registered actually make it to the polls on Election Day. As a whole, this small subset of adults known as “likely voters” is much more supportive of Republicans than the general public is.

For example, a recent AP-GfK poll (PDF) of adults has gotten some press because it shows Democrats with a large seven-point lead in the generic ballot, ahead of Republicans by 46 to 39 percent. That’s actually not good news for Democrats. In an October 2008 poll, AP-GfK found that Democrats had a 13-point lead among all adults. But that translated to only a six-point lead when the poll screened for likely voters.

A wide range of polls shows that all adults lean more Democratic than registered voters, and registered voters are often more Democratic leaning than likely voters. An Ipsos poll (PDF) from early May had the generic ballot at 46 percent for Democrats and 39 for Republicans among all adults but among registered voters, the lead was a much narrower 45-42. The average of other Ipsos polls from this cycle shows Democrats doing about 5.5 points better among all adults than among registered voters.

“The Economist” has helpfully broken down the results from its most recent generic ballot poll into all adults, registered voters and likely voters.

All Adults
Rep 39 – Dem 44: D+5
Registered Voters
Rep 43 – Dem 47: D+4
Likely Voters
Rep 47 – Dem 46: R+1

Republicans have a significant advantage among likely voters. Several points in the generic ballot could translate to making the difference in several dozen House races and control of the chamber. The majority of American adults may not support Republicans in November, but it’s possible that Republican Congressional candidates could gain the plurality of votes cast.

If I were a Democratic operative looking at these numbers, my first act of business after taking control of the Presidency, the House and the Senate would not have been the stimulus or health-care reform. It would have been a series of voter-access measures.

There are many ways to increase the number of adults registered to vote and to get a greater percentage of people registered to turn out at the polls. Same-day registration, automatic registration of high school seniors and default registration when people access any local social service or DMV are ways to increase the number of registrations. Making Election Day a holiday, opening polling locations for 24 hours, increasing access to early voting and even making voting compulsory (as some democracies, like Australia, do) could greatly help to improve turnout.

Looking at these poll numbers, the lack of a full-court press by Democrats to push through legislation that would make it easier to register and vote seems like one of the worse strategic errors in contemporary politics.