Speaking at a conference about first ladies at Southern Methodist University on Monday, Mrs. Bush called the 2012 presidential race “the worst campaign I’ve ever seen in my life.”

“I hate that people think compromise is a dirty word,” she said. “It’s not a dirty word.”

Though her husband, former President George H. W. Bush, unofficially endorsed Mr. Romney in late December, calling him “the best choice for us,” Mrs. Bush did not ramp up her public support for Mr. Romney until recently.

But now that she’s in, she’s been a frequent presence on television, in the newspapers and even on the phone to voters in crucial Super Tuesday states. On Sunday, the Romney campaign began a round of robocalls recorded by Mrs. Bush in Ohio and Vermont.

In the call, Mrs. Bush directs voters to a phone number and e-mail address for the Romney campaign headquarters in their state. When recording the call, she omitted an anti-Obama line in the script, according to her spokesman.

But negative campaigning is not new for the Bush family. Mrs. Bush made her famous “rhymes with rich” line, and her husband ran the Willie Horton ad against Michael Dukakis in 1988.

On Monday evening, Mrs. Bush continued her campaign for Mr. Romney in a phone call with Fox News. Mrs. Bush said that her ties with the Romney family dated to the days of Mr. Romney’s father, George, a former auto executive who became the governor of Michigan.

“We’ve known the Romneys for a number of years,” she said. “I knew the mother and father. I’m so old, I probably knew the grandmother and grandfather. In any case, we loved them. George Romney started really the inspiration for the points of light. He just was a wonderful giving man, and his son is the same.”

Though she went out of her way to point out that she, like her husband, has not officially endorsed Mr. Romney (“I haven’t endorsed anyone, yet of course you think I have obviously,” she told the host), her visible role supporting Mr. Romney certainly seems a lot like an endorsement.

“I’m sort of sick of people tearing him down, because I think he’s a great man, would be a great choice,” she said.

“I’m worried about this campaign, because it’s too ugly, I really don’t like it,” she said on Fox News. “I’m very much for Mitt Romney, as I guess is my husband — I’m endorsing him for George.”

“There I go again,” she added.