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).

Team Romney understands this and is deploying Ann more often and on more platforms, from TV and radio shows to public rallies and campaign videos. “She rounds him out,” Romney adviser Tom Rath told Politico. “You live with a guy for 40 years and you’re qualified to speak to what kind of man he is.” Adds GOP strategist Alex Castellanos: “His link to the base doesn’t come from ideology. It comes from family values channeled through Ann.”

Wives can be a tricky business. Bill Clinton was not particularly successful selling Hillary as co-president, and her stewardship of health-care reform in the White House was a disaster. (Once she stepped out of Bill’s shadow — first in the Senate and then in Obama’s Cabinet — she’s been America’s most-admired woman for the last 10 years, according to Gallup.)

Laura Bush (still the sixth most-admired woman) is a better model for Ann Romney. She highlighted her husband’s human side by helping George overcome his penchant for alcohol and then softened his warrior image in the White House by becoming “comforter in chief.”

Michelle Obama is a critical part of her husband’s re-election effort. One tipoff is a widely circulated online ad featuring both parents and their two daughters under the slogan, “Help the Obamas Stand Up for Working Americans.” The message: We’re a team, a family, and that helps us understand your family.

When Romney tries to talk about personal things, he often screws up. He has trouble identifying with working families because, unlike the Obamas, he’s never lived in one. When he told voters he once worried about getting a “pink slip” he was totally unbelievable, since he had an MBA from Harvard and a father who ran an auto company.

Nor has Romney ever battled the kind of personal demons that bedeviled George Bush. He’s “too perfect,” as one voter put it. His personal life seems totally unblemished by trouble or tragedy. That’s why voters see him as insulated and insensitive. Can you really understand suffering if you’ve never felt it yourself?

But Ann Romney has not been protected from life’s trials. She’s had two major illnesses — breast cancer and multiple sclerosis — and when her husband talks about her, his feelings start to show through.

At a town hall in Wisconsin, a question about his religion prompted Romney to discuss the burdens most people carry, which led to his wife’s health: “My good wife, I mean, you see her, she’s beautiful, she’s energetic, articulate, but you know she has MS, and she also had to fight breast cancer. And I watched her as a person with great strength and capacity. You don’t always see the things that are happening in peoples’ lives.”

It’s when she talks about her illness that Ann is most effective as a character witness for her husband. In a campaign video, she discusses the time she was diagnosed with MS: “I was frightened; Mitt was frightened. But I needed him desperately. He was so reassuring and so loving.”

Now, he needs her. Desperately.

(Steve and Cokie Roberts can be contacted by email at stevecokie@gmail.com)