Poli-Sci Perspective is a weekly Wonkblog feature in which Georgetown University's Dan Hopkins and George Washington University's Danny Hayes and John Sides offer an empirical perspective on the issues dominating Washington. In this edition, Hayes looks at the change in public opinion on the death penalty. For past posts in the series, head here.

When Maryland Gov. Martin O’Malley (D) signs a bill abolishing the death penalty, the state will be just the latest to wipe capital punishment off its books. Seventeen other states and the District of Columbia have outlawed it, five since 2007. Legislators in Delaware this week announced a bill to do the same.

Many of the states that have abolished capital punishment – including the recent cases of Connecticut, Illinois, New Mexico, New York and New Jersey – are, not surprisingly, among the country’s most liberal.

But the erosion of public support for the death penalty has occurred across the nation, in large part because Americans are conflicted. Many believe capital punishment is justified, but they worry that innocent people might be executed. And as the political debate has in the last two decades focused on wrongful convictions and death row exonerations, Americans have increasingly come to evaluate the death penalty in terms of its potential unfairness.

Although a large majority of Americans – 63 percent in a December USA Today/Gallup poll – say they favor capital punishment, many nonetheless harbor misgivings.

For instance, in a 2001 Washington Post-ABC News survey, 72 percent of Americans agreed with the statement that “the death penalty is fair because it prevents killers from killing again.” Likewise, 60 percent said the punishment was fair “because it gives satisfaction and closure to the families of murder victims.”

But many respondents simultaneously said capital punishment was not just. Asked whether they agreed that “the death penalty is unfair because it’s applied differently from county to county and state to state,” 63 percent said yes. And 68 percent concurred that it is unfair “because sometimes an innocent person is executed.”

Marylanders, too, appear ambivalent. In a Washington Post poll of Maryland residents last month, 54 percent said they supported the death penalty. But 61 percent said they believed it doesn’t reduce crime, and just 43 percent said they believed it was applied fairly. Nearly one-third said capital punishment has been applied unfairly, and a quarter said they didn’t know.

Such ambivalence is not unique to the death penalty. Work by political scientists has shown that people often possess multiple, and competing, attitudes about lots of public policy issues. After 9/11, for example, Americans expressed discomfort with law enforcement activities that seemed to infringe on civil liberties, but many also believed that such activities were justified to protect the nation from terrorism.

But precisely because ambivalence makes people more susceptible to changing their minds, the reframing of the death penalty debate has significantly reduced support for capital punishment.

In "The Decline of the Death Penalty and the Discovery of Innocence," political scientists Frank Baumgartner, Suzanna DeBoef and Amber Boydstun found that since the mid-1990s, news coverage of the death penalty has increasingly focused on exonerations and wrongful executions. In earlier eras, the debate in the media was more frequently about other issues, such as capital punishment’s constitutionality or cost.

This shift in media coverage, which has highlighted problems in the death penalty’s application, has encouraged the public to evaluate capital punishment in terms of fairness, especially the potential for innocent people to be sent to death row. As a consequence, Baumgartner, DeBoef and Boydstun find that along with a decline in the U.S. murder rate and other high-profile events (such as former Illinois governor George Ryan’s (R) 2001 mass commutation of death row inmates), negative news drove down support for capital punishment.

How much did public opinion move? By one measure, 86 percent of Americans in 1995 said they favored the death penalty for people convicted of murder. But by 2006, just 70 percent did.

These developments have not, of course, erased many Americans’ view that the death penalty may be justified. A May 2012 Gallup survey showed that 58 percent believed capital punishment is “morally acceptable.” But as the “innocence frame” has come to dominate the debate, Americans belief in the morality of the death penalty has come to matter less, and the system’s capacity to make an irreversible mistake has come to matter more.