The candidate's position was clear and direct: "I have no purpose to interfere with the institution of slavery," he said. "I have no lawful right to do so."
But as president, he declared the slaves free, at least in some states, by executive order.
Abe Lincoln, flip-flopper? Or did the Great Emancipator simply evolve?
Ronald Reagan was for a woman's right to choose an abortion before he was against it, yet his shift was widely viewed as principled evolution, not craven politics.
Mitt Romney has made much the same journey, only to find himself battered by skepticism about his motives and principles. After months of attacks about position-switching from his Republican primary opponents, Romney was markedly restrained in responding to President Barack Obama's announcement this month that he was flipping from opposing same-sex marriage to endorsing it.
In politics, a mind can be a terrible thing to change.
Attack ads featuring spinning weather vanes and double-talking candidates have become a staple of American campaigns, but consultants who make those ads say the flip-flopper charge doesn't always stick — and no one has a formula to predict exactly when it will.
But there are reasons why some politicians' changes of heart strike voters as evidence of duplicity and others are accepted as the result of reasoned reconsideration — and America's cultural divide on social issues might explain why certain flip-flopper charges hit home.
"Voters tend to be fair-minded about this kind of thing," said Geoff Garin, a longtime Democratic pollster and strategist. "Voters look for obvious motive, for cravenness. You get extra demerits for being a repeat offender."
Was it flip-flopping, evolving or adapting to altered circumstance when Obama changed direction about closing the Guantanamo Bay detention camp for suspected terrorists, or about the wisdom of standing by Egyptian President Hosni Mubarak, or about whether Obama campaign officials would appear at fundraisers for super PACs?
This month, after many months of saying that his position on same-sex marriage was "evolving," Obama finally announced he had completed his shift from "I believe that marriage is the union between a man and a woman," as he put it in 2008, to "I think same-sex couples should be able to get married."
The resulting criticism focused not on the honesty of his belief but on his timing — was he just coming out with his support to win contributions from wealthy gay donors? Obama largely escaped the flip-flop charge because, Garin said, "lots of people assumed this was really Obama's position all along."
In contrast, on abortion, Romney had sent mixed signals for most of his political career. He explained his original pro-abortion-rights position on Fox News last year as more a matter of strategy than of principle; when he settled on his original position in Massachusetts, Romney said, he decided that "I'm just going to say I will support the law and preserve the law as it exists."
"Romney is more suspect because he's in the serial offender category," Garin said, noting that on issues such as abortion, gay rights and gun control, the Republican candidate often defends his current positions by saying they aren't that different from his past stances.
That's a tactic that hardly ever works, as evidenced by John Kerry's widely lampooned statement in the 2004 campaign that "I actually did vote for the $87 billion [for the Iraq war] before I voted against it." The flip-flopper tag proved especially sticky in Kerry's case, said longtime Republican advocate Morton Blackwell, because of the public perception of contradictions in Kerry's history as both a veteran and an antiwar activist in the Vietnam era.
When President George H.W. Bush famously reneged on his pledge at the 1988 Republican National Convention — "Read my lips: No new taxes" — that did not fall into any pattern of flips. Indeed, polls at the time showed Bush to be widely trusted. Still, the flip was a factor in Bush's tumble from extraordinary popularity to eventual defeat in his bid for re-election.
The Bush tax shift illustrates another theory about why some changed positions can be devastating: If you take a principled position, the public wants to see you stick with it, even if it's going to hurt.
"You should never completely trust someone until you have observed that person sticking with a losing side when they could have won by switching sides," said Blackwell, who teaches thousands of young Republican political wannabes at his Leadership Institute in Arlington, Va. "It's that willingness to lose that tends to separate the principled from the opportunists."
If the flip-flopper label seems to have been applied more frequently in recent years, that's in part because of more aggressive political ads, an accelerated news cycle and news coverage that is ever more assiduous about hunting for candidates' inconsistencies, said Brendan Nyhan, a political scientist at Dartmouth College who studies political scandals and spin.
In addition, as Thomas Mann and Norman Ornstein argue in their new book, "It's Even Worse Than It Looks," the increasingly ideological nature of both political parties has given rise to a quest for purity in which a changed mind makes a politician untrustworthy.
Both Obama and Romney "have changed lots of positions," Nyhan said. "But the press is going to punish Romney, because there's this perception of him as inauthentic, like with Al Gore: The elite press had a very cynical view of him as someone who just said what the public wanted to hear, and they just punished him and punished him for it."
But what strikes one voter as inauthentic might be a sign of intellectual flexibility to another. That's where a new study by David Barker and Christopher Carman suggests a way to predict which flips will bother which voters. The political scientists devised an experiment in which voters were asked to choose between these two hypothetical candidates' pitches:
Candidate A: "My only priority is working hard to SERVE the people in this district. I will be your MOUTHPIECE. . . . You see, I'm not so arrogant as to think I know better than you what's best for you. That's why I'll really LISTEN to you, and the folks in Washington are going to hear YOUR VOICE, for once."
Candidate B: "My only priority is working hard to DO WHAT'S RIGHT for the people in this district. I will be your ADVOCATE. . . . You see, I believe that leaders should LEAD, guided by firm PRINCIPLES that don't change every time the polls do. The folks in Washington are going to see what that looks like, for once."
These candidates represent a classic argument in political philosophy between the view of John Stuart Mill, the British philosopher who said that democratically elected officials should reflect constituents' views, and that of Edmund Burke, the Irish-born political thinker who argued that we elect representatives with strong values so they will follow their principles.
Voters who preferred Candidate B — Burke's view — responded much more negatively to candidates who changed their minds on issues, said Barker, director-designate of the Institute for Social Research at California State University at Sacramento. Those voters generally prefer conservative Republicans and are more likely to rely on religious faith to guide their political choices.
Voters who preferred Candidate A — Mill's view — were much more accepting of candidates who flipped on issues. These voters, mostly drawn to more liberal, Democratic candidates, tend to be more secular and believe that as the people's views shift, so should their leaders'.
Blackwell, the GOP activist, agreed that faith-driven voters are more likely to demand consistency. In 2004, religious voters were drawn to George W. Bush, the steadfast "decider," over John Kerry, a malleable rationalist, he noted.
"Kerry epitomized the stereotype people have of wishy-washy politicians," said Garin, the Democratic strategist. "With Bush, people said, 'I may not agree with him, but we're in a war and I know he's going to be firm in his conviction to protect America.' "
Barker said his theory explains why John McCain, perceived as a maverick, defeated Romney for the 2008 GOP nomination: "McCain was seen as someone who would stand on principle, but conservative voters didn't trust Romney because they saw him as too willing to change his views."
But there are also elections where the candidates' styles don't match Barker's categories so neatly, and this year is one of them.
Both Obama and Romney are given to arguing that they are following the views of the people, and for Romney, that has caused problems, Barker said: "In Massachusetts, where there aren't many evangelicals, Romney can say he personally opposes abortion but supports keeping it legal because that's what the people want. But in South Carolina, there sure are a lot of evangelicals, and in their view, you're suddenly changing your mind about something they consider murder, and they find that kind of repulsive."
Blackwell said he teaches budding politicians "that if you establish for yourself a set of principles and stick to it, people will trust you." He said the only politician in recent memory who could not be accused of ever changing a position was Jesse Helms, the late Republican senator: North Carolina voters, many of them religious conservatives, "knew Jesse Helms would do exactly what he said, and they could trust him."
In the end, voters are especially willing to accept a shift in politicians' positions "if it's an issue where the public has evolved in its own thinking," Garin said.
Of course, whether independents will accept Obama's new stance on marriage remains unknown, just as it's unclear whether conservatives, who spent much of this year's primary season searching for an alternative to the right of Romney, will be enthusiastic about his campaign. Theories about why voters behave as they do are always more useful in understanding the past than predicting the future.