Americans were unhappy about many issues as 2012 began. In one area, though, contentment reigned. By a margin of 50 to 45 percent, a Gallup Poll reported, the public felt “satisfied” with the nation’s policies on crime.

It was a well-founded sentiment. In 2010, Americans were less than a third as likely to be victimized by violent crime as they had been in 1994; the murder rate had declined by roughly half. Today we are approaching the low murder rates of the 1950s.

For the Republican Party, this is a triumph — and a disaster, as the 2012 election results proved.

It is a GOP triumph, because the enormous decline in crime over the past two decades coincided with the widespread adoption of such conservative ideas as “broken windows” policing and mandatory minimum sentences.

Whether such policies actually caused the crime decline is a separate, and much-debated, social-science question. The important thing is that many people believe that they did. As a result, conservative crime doctrine remains dominant in politics, with the two parties differing mainly over how to control and punish unlawful conduct most cost-effectively.

Hence the 2012 disaster for the GOP. Beginning with Richard Nixon’s “law and order” campaign for president in 1968, Republicans pretty much owned the issue. Fear of street crime — and its association, accurate or not, with post-’60s moral license, liberal Democratic policies and the rise of an urban black population — converted many a white working-class Democrat into a Republican.

The GOP advantage on crime contributed to Ronald Reagan’s election in 1980, George H.W. Bush’s defeat of Michael Dukakis in 1988 and the GOP takeover of the House of Representatives in 1994.

When Gallup asked voters in January 1995 to name their top priority for the new Congress and President Clinton, 78 percent responded “reducing crime.” Given the murder rate at the time — 9.0 per 100,000 population — this was understandable. Sixty-six percent named “reforming the welfare system.”

Clinton got the message. In 1996, he signed the Anti-Terrorism and Effective Death Penalty Act, a main purpose of which was to limit death-row appeals. And, of course, he signed a historic welfare reform measure.

As the first Democratic president since Clinton, and the first African American one ever, Barack Obama has done essentially nothing to reverse Clinton’s crime and welfare policies. He signed a bill reducing the disparity in penalties for crack and powder cocaine possession under federal law, a modest reform that enjoyed wide Republican support in Congress.

This year, bureaucrats in Obama’s Department of Health and Human Services suggested waivers of the work requirement for some welfare recipients. Mitt Romney’s campaign accused the president of trying to “gut” welfare reform.

But Romney’s effort to revive the partisan divide on welfare failed as the administration quickly renounced any dilution of the work requirement — and as it became clear to voters that the cap on total welfare spending and other key aspects of reform would remain intact.

Indeed, Obama’s assimilation of conservative doctrine extended even to the war on terrorism, an area with which 72 percent of the public pronounced itself satisfied in last January’s Gallup Poll. Closing Guantanamo is out; drone strikes on al-Qaeda suspects are in. After four years of the Obama war on terror, you could almost summarize the two parties’ policies this way: Republicans waterboard, Democrats kill.

It’s true, as many commentators have noted since Nov. 6, that liberals seem to have the upper hand in the culture wars, after years of losing to the GOP. The 2012 electorate favored liberal positions on abortion, gay rights and the role of women in society.

We’ll never know whether 2012 would have played out the same way if crime had staged a comeback during the recession, as many expected. Certainly in the past, crime was as important to the Republican brand as abortion and gay rights, if not more important.

Safer streets, though, have blunted what was once a sharp wedge issue, and, perhaps, freed the electorate to consider social and moral issues in a different light.

In the crime-ravaged ’70s and ’80s, Clint Eastwood’s “Dirty Harry” Callahan acted out Middle America’s fantasy of a no-holds-barred war on crime.

By the time an elderly Eastwood appeared at the 2012 GOP convention, though, violent crime was a fading specter. And when he led the crowd in a chorus of “Go ahead, make my day,” it was history repeating itself as farce.

He should have said, “We need a new issue.”