Far more Americans favor Democrats over Republicans. For decades, the number of Americans identifying as Democrats or calling themselves independent but leaning Democratic has far exceeded the share of Republicans and Republican leaners. That gap has persisted, even in landslide Republican years like 1984 and 1994.

So why don’t Democrats perform better in national elections? Why have Democrats won only four of 10 presidential races since 1972?

A new report for Third Way, the moderate Democratic group, posits an answer: the ideological disconnect between liberal party activists and moderate party voters. In “Family Feud: Democratic Activists v. Democratic Voters,” Todd Eberly, a political scientist at St. Mary’s College in Maryland, examined data from the American National Election Studies and focused on the striking divide among Democrats.

In the 10 presidential elections since 1972, Democratic activists — those who attended a campaign event and donated money — rated themselves an average of 3.06 on a 7-point liberal-to-conservative ideological scale, with 4 being “moderate.” By contrast, those who merely identify as Democrats or lean that way were significantly closer to the center, an average of 3.77.

This “ideological gulf,” Eberly argues, coincides with — and helps explain — decreased party loyalty. Since 1970, Democratic-leaning independents have increased from fewer than one in five members of the Democratic coalition to one in three. This shifting composition makes a difference.

First, the Democratic-leaning independents are far more likely to switch loyalties and vote Republican than are their pure Democratic counterparts. This may seem obvious, but consider: Republican leaners were far less likely to defect than were Democratic leaners.

For example, in the 2002 House election, 46 percent of those who had identified themselves as Democratic-leaning independents two years earlier voted for Republicans; just 26 percent of Republican-leaning independents switched to vote Democratic.

Second, the Democratic-leaning independents have different views than those who call themselves Democrats. As Eberly reports, they are “less supportive of government intervention in the economy, more likely to believe that the government has gotten too involved in things people should do for themselves, and express higher levels of support for cutting Social Security spending.”

Eberly’s conclusion: “There may be more money and passion among activists on the left, but there aren’t enough voters there to secure consistent electoral victory for Democrats. The true wealth of voters in the Democratic coalition resides in the vital political center and that’s where the Democratic Party will find the path to sustained electoral dominance.”

The key here is the word “sustained.” There is a long-running debate among strategists in both parties about whether the path to electoral success lies in turning out the base or appealing to independents.

In a recent paper for the Center for American Progress, Ruy Teixeira and John Halpin make the demography-is-destiny case — that the rising share of minority voters, in particular, “clearly favors Democrats.” This argues, they say, for a strategy of energizing the progressive base, not questing fruitlessly after independents.

Perhaps so, in the short run. But I’d argue that some of President Obama’s recent choices, such as rejecting the Keystone XL pipeline or his original stance on mandating that religiously affiliated institutions provide contraceptive coverage, risk alienating at least as many voters as they inspire.

Over time, however, the base approach assumes that minorities, especially Latinos, are hard-wired to the Democratic Party, and it fails to take adequate account of the growing loosely connected component of the Democratic coalition.

Then again, Republicans seem determined to save Democrats from themselves. “Republicans should not look to this report as good news for the GOP,” Eberly wrote. “If the GOP continues to trek to the right, they will reach a point where moderate Democrats no longer view the GOP as an acceptable alternative in elections.”

Eberly’s data show that Republican activists and Republican voters are much more ideologically compatible. The Democrats’ activist-supporter ideology gap is more than twice that of Republicans.

And where Democrats, both activist and everyday voters, have stayed in essentially the same ideological place over the years, Republican activists and Republican supporters have shifted markedly rightward. For example, Republican activists have moved from close to moderate (4.53) in 1972 to decidedly conservative (5.50) in 2008.

In short, Democrats are a distinctly purple party with blue leadership. Republicans are a uniformly red party becoming redder by the year. Those clashing palettes frame the parties’ very different challenges.

Democrats need to align liberal activists with more moderate supporters. Republicans must attract enough moderate voters without alienating an increasingly conservative base.

The Democratic mission is more achievable — if, that is, the party chooses to accept it.