Voters don’t make decisions based on a candidate’s spouse. But when a man runs for president, his wife plays an important role as validator, as character witness, testifying to the human qualities behind the poll-tested speeches and slickly produced videos.

No one knows him better, so no one can speak with more authority. As Rick Santorum’s wife, Karen, put it during the Wisconsin primary: “You always hear the other side of the candidates, and I think it’s also just really nice when the spouses give a more personal perspective, a window into our lives.”

Karen probably won’t get the chance to talk about her husband much longer; unless Mitt Romney gets creamed by his own campaign bus, Santorum won’t be the Republican nominee. But America will be hearing a great deal from Ann Romney this fall.

Her husband has tried — and failed repeatedly — to come across as a warm, approachable person who understands ordinary folks and their problems. That’s why his wife has to make the case for him.

Most people don’t vote based on a checklist of issues. They want an emotional connection with their candidate. Feelings are as important as facts. And Romney’s compassion gap is particularly damaging among female voters. In the latest USA Today/Gallup poll of 12 swing states, President Obama leads Romney by 18 points among women (and only one point among men).