For more than a decade now politicians and pundits have talked about a polarized electorate — red-state conservatives and blue-state liberals. Both parties focus on turning out their “base.”

Commentators often overlook evidence that millions of Americans don’t line up neatly in these red and blue boxes. Many of them are conservative on fiscal issues like tax and spending but moderate or liberal on “social issues” like abortion rights and gay marriage. (And some of course are liberal — that is, American-style welfare-state liberal — on economic issues and conservative on social issues.) You might say those voters want government out of their personal lives and out of their businesses.

The Gallup Poll finds that about 20 to 23 percent of Americans fall into the fiscally conservative, socially liberal — or libertarian — group. Using tighter criteria, David Kirby and I have found in a series of studies that about 15 percent of Americans can be classified as libertarian voters.

The libertarian voters have often given about 70 percent of their vote to Republican candidates, though they’ve shown more willingness than most Americans to vote for independent candidates such as John Anderson in 1980 and Ross Perot in 1992 and 1996. They preferred George W. Bush over Al Gore by 72 to 20 percent. But the election of 2004 saw a dramatic swing away from the Republicans, with libertarian support for Bush dropping from 72 to 59 percent, while support for the Democratic nominee almost doubled to 38 percent. Bush’s record on war, spending, entitlements, and social issues certainly pushed libertarians away.

That weakened support for Republicans lasted into the 2006 congressional elections. In a survey conducted for the Cato Institute by the Zogby firm, 59 percent of libertarians voted for Republican candidates for Congress, and 26 percent voted for Democrats.

Compared to the previous off-year election in 2002, that was a 24-point swing to the Democrats.

Data from the American National Election Studies show an even stronger swing away from Republicans from 2002 to 2006. The margin for Republican House of Representatives candidates among libertarians dropped from 47 to 8 points, a 39-point swing. And the margin for Republican Senate candidates dropped from 59 to 4 points over that period, a 55-point swing. Those are far bigger swings than will be found among conservatives, the religious right, union members, or other more-studied voting groups.

In 2008, fearing unified Democratic Party control of the federal government, libertarians voted heavily against Barack Obama. And they formed a vital part of the tea party movement that shook up American politics in 2009 and 2010.

This year Rep. Ron Paul rallied millions of libertarian voters in the Republican primaries, though he fell short of the nomination. Where those voters go in November will have a huge impact on the results. A poll in 2008 showed that the more a voter liked Ron Paul, the less likely he was to vote for Republican nominee John McCain. The Mitt Romney team should take note of those results. A lot of Ron Paul’s voters are considering staying home or voting for Libertarian Party nominee Gary Johnson. Romney needs to find a way to keep them in the Republican camp, and he doesn’t seem to be making the sale.

You might think Obama would have trouble holding on to civil-liberties voters after his drone attacks, stepped-up deportations of immigrants, assassinations of American citizens, and unconstitutional war in Libya.

But there seems no evidence that civil libertarians have wavered in their support for the president.

A Washington Post poll in August found that about a third of all voters call themselves independents. But “nearly two-thirds of Americans who describe themselves as independents act very much like partisan Republicans or partisan Democrats.” Another quarter have a low interest in politics and are unlikely to vote.

Which leaves 13 percent of the independents, or about five percent of the total electorate, designated by the Post as “Deliberators,” people genuinely open to candidates of both parties. More than half of them say they’ve voted for candidates of both parties in the past, and right now Obama and Romney claim only about 30 percent each.

And what do those voters want? Well, only 26 percent of them like President Obama’s health care plan, and 64 percent say they prefer “smaller government with fewer services” to “larger government with more services” — and 63 percent favor gay marriage. So they seem like classic “fiscally conservative and socially liberal,” or libertarian, voters.

A new Reason-Rupe poll finds Romney with 77 percent of the libertarian vote, better than I would have predicted. It probably reflects libertarian enthusiasm for budget hawk Paul Ryan and strong rejection of Obama’s health-care takeover and trillion-dollar deficits. But Romney aides should note: when the poll mentioned Gary Johnson as a choice, he pulled 14 percent of the libertarians.

Both parties are working hard to turn out their base votes. They should devote equal effort to appealing to the independent libertarian-leaning voters who are still undecided.