Compelling new evidence reveals that Congressman-Elect Mark Sanford won last week’s special election victory because voters who made their choice in the last week of the campaign broke overwhelmingly in his favor – and the number one reason they cited was the need to repeal ObamaCare.

The evidence comes in the form of a post-election survey fielded the day after the election by GEB International, an international survey research firm commissioned by Independent Women’s Voice, a 501(C)(4) organization that spent approximately $250,000 to help Sanford win. The poll surveyed 400 respondents who had cast ballots in last Tuesday’s special election in South Carolina’s First Congressional District, and has a margin of error of +/- 5 percent at the 95 percent confidence interval.

According to the GEB International survey, 15 percent of the voters in the special election – roughly one in seven – did not make up their minds until the last week of the campaign. Sanford won his surprisingly lopsided 54-45 percent victory over Democrat Elizabeth Colbert Busch because these late-deciding voters broke 70-22 percent for him – an unheard-of feat for a scandal-tarred virtual incumbent. That +48 point advantage in the final week simply swamped Colbert Busch.

Just who were these late-deciding voters? Republicans and Independents, mostly. According to the survey, 90 percent of Democrats made up their minds before the last week. And Democrats as a whole voted 92-4 percent for Colbert Busch. But fully 20 percent of voters who said they almost always vote Republican waited until the last week to decide – and they broke 88-8 percent for Sanford.

Conventional wisdom in this race said the key to a Colbert Busch victory would be her ability to pull disaffected Republicans and Independents across the line to vote for her. Her strategy was to make the race all about him and his personal troubles. If she could pull a quarter of the GOP and Independent vote, she had a good shot at winning.

Sanford’s communications task, not surprisingly, was the exact opposite – take control of the campaign conversation and move it off the personal stuff and get it onto more favorable terrain, to wit, legislative and political issues. Survey research and message testing conducted by IWV two weeks before the election showed Colbert Busch’s positions on major issues – ObamaCare, unions, raising the debt ceiling – were well outside the mainstream of the SC-01 electorate. Despite the fact that he was trailing Colbert Busch by 9 points at the time, his road to redemption was clear: If the campaign conversation could be moved onto issues terrain, Sanford had a path to victory.

That’s where the IWV Independent Expenditure campaign and other communications came in. IWV knew that because of Sanford’s high personal negatives, he was the wrong messenger to carry a negative fact-based message about Colbert Busch. But IWV, as a credible third-party messenger, could deliver that necessary message, and inform SC-01 voters about the troubling aspects of Colbert Busch’s positions on these key issues.

(Sanford’s campaign understood this, too – they ran only one attack ad on TV against Colbert Busch during the campaign, and took it off the air ten days before the election, so they could close with a well-produced positive ad featuring Sanford himself talking directly to the camera.)

Consequently, IWV developed and deployed a communications program that involved broadcast and cable TV advertising; live and recorded phone calls; and newspaper advertising. Each messaging vehicle delivered a simple, fact-based message educating voters on Colbert Busch’s positions on the issues.

That communications program focused on four simple messages about Colbert Busch (each of which had been shown in a message test had the capacity to move numbers against her): She had refused to sign a pledge to repeal ObamaCare; she had accepted campaign cash from the PAC of the labor union that had tried to block Boeing; she had taken $43 million in stimulus money, but only created or saved 134 jobs; and she would be a vote for a higher debt ceiling.

The IWV communications campaign worked. Among voters who decided more than a week before the election, Colbert Busch’s favorable-to-unfavorable comparison was 45/33 percent, for a +12 percent positive; but among those who decided in the last week (when the IWV advertising was running), she was upside down, at 34/41 percent favorable-to-unfavorable. That -19 point swing sealed her fate.

While there was no killer issue against Colbert Busch – each of the issues worked together to reinforce one another and drive her negatives up – it turned out there was a silver bullet in terms of persuading the remaining undecideds to vote for Sanford: ObamaCare, and Colbert Busch’s refusal to sign a pledge to work to repeal it.

According to the GEB after-action survey, 19 percent of the electorate said repealing ObamaCare was the single biggest reason to vote for Mark Sanford. That was the number one response given to the question. Among those who decided in the last week, that percentage rose slightly, to 20 percent. Among self-identified Ticket Splitters, that number was 28 percent. Among those who said they remembered receiving a live phone call, the number was 24 percent; among those who said they remembered receiving one of IWV’s quiz calls – in which call recipients are invited to test their knowledge by taking a quiz on the issues – that number jumped to 30 percent, more than 50 percent higher than in the survey sample as a whole.

Perhaps more compelling for candidates who seek to repeal ObamaCare, female voters were more likely to cite ObamaCare as the number one reason for voting for Sanford than were men – 15 percent for men, 23 percent for women. In virtually every congressional district in the nation, female voters outnumber male voters; pro-repeal candidates would be wise to seek female support by highlighting their opposition to ObamaCare.

Despite President Obama’s recent insistence that everything seems to be going fine with the implementation of his health care law, the electorate seems to think otherwise. ObamaCare was a major issue in the 2010 election cycle, and likely would have been in 2012, too, if Republicans had nominated for President any candidate other than Mitt Romney. Mark Sanford’s victory shows Republicans that the issue remains as compelling as ever.

The question now is, will Republicans listen?

Bill Pascoe is a partner in Antietam Communications, and counsels Independent Women’s Voice on its political and communications strategies.