Among political urban legends, one of the more persistent is "base elections"—the notion that successful campaigns can rely simply on turning out a party's hard-core supporters.
Nonsense. The party that wins independents wins Congress. Energizing core supporters is necessary but insufficient.
Democrats took control of the House in 2006 by winning independents 57% to 39%, according to national exit polls. They kept control in 2008 by carrying independents 51% to 43%.
In 2010, Republicans won back the House by sweeping independents 56% to 37% and retained their majority two years later by taking independents 51% to 44%. Independents made up between 26% and 29% of voters in those elections while constituting between 34% and 40% of the electorate.
The independent voter will be even more important in this year's midterm. Last week Gallup announced that in 2013 the percentage of the electorate that self-identified as independent rose to 42%, the highest share since it began conducting surveys by phone a quarter century ago. The percentage of the electorate that was independent was 40% in 2012 and 36% in 2008.
The alarming news for Republicans in the Gallup survey is that the percentage of voters who identify with the GOP is the lowest since 1983—25%, a drop of three points from 2012. The Democratic share of the electorate is 31%, where it's been since 2010 (down from a quarter-century high of 36% in 2008).
While President Obama's approval rating took a dive last year, the GOP didn't gain as much of an advantage as it might have. Independents strongly disapproved of last October's government shutdown and blamed Republicans. The more dysfunctional Washington appears, the more independents blame everyone.
The Republican-controlled House appears unlikely to provoke another shutdown; to do so would be insanity. But avoiding disaster isn't enough. Strengthening their House majority and taking control of the Senate will require Republicans to present a constructive conservative agenda on big issues that win over independents.
Fortunately for Republicans, independents this year look more like Republicans than Democrats on those issues and are therefore more "gettable" by GOP candidates.
A Jan. 7 Quinnipiac University poll underscores this. Forty-seven percent of Democrats are very or somewhat "satisfied with the way things are going in the nation today." But only 19% of independents and 12% of Republicans say the same.
On Mr. Obama's "handling his job as president," 81% of Democrats approve while only 35% of independents and 9% of Republicans do. Among Democrats, 86% believe Mr. Obama is honest and trustworthy and 10% don't. Fifty-three percent of independents and 84% of Republicans say he is not.
More than three-quarters of Democrats (76%) approve of the president's handling of the economy, compared with only 36% of independents and 7% of Republicans. Quinnipiac found 71% of Democrats approve of the president's handling of the federal budget while 66% of independents and 89% of Republicans don't.
Then there's ObamaCare. Seventy-one percent of Democrats approve of the president's handling of health care and 80% support his Affordable Care Act. Independents give Mr. Obama a miserable 31% approval on handling the issue and 35% approve of ObamaCare. Only 6% of Republicans approve of Mr. Obama's handling of health care while 9% approve of ObamaCare.
The Quinnipiac Poll found that independents would be less likely to vote for "a candidate for Congress" who "supports Mr. Obama" by a better than 2-1 margin (44% to 20%). It is likely that independents look even more like Republicans in the seven red and half dozen purple states with Democratic senators where control of the upper chamber will be decided.
The reason? Independents are not parked in the middle between the two parties. They are spread out across the political spectrum. Independents tend to look like the rest of the voters in their state or district but with much less party loyalty, political interest or civic engagement and, compared with partisans, lack a cohesive, organized belief system.
This could be a Republican year if the GOP understands how important independents are to deciding elections and cultivates them. Criticizing Democrats who have loyally backed Mr. Obama is part of what's needed. But criticism comes naturally and easily. A comprehensive agenda focused on the economy, spending, deficits, health care and energy is more difficult. And more important.
A version of this article appeared January 16, 2014, in the U.S. edition of The Wall Street Journal, with the headline Independents Will Decide the 2014 Elections and online at WSJ.com.