John McCain has had cancer four times and would be the oldest president ever elected, yet, according to two left-leaning groups, the 72-year-old GOP nominee hasn’t been open enough with the public about his medical records.

In a new ad (subscription) scheduled to begin running nationwide Thursday, Democracy for America and Brave New PAC pairs doctors’ grim diagnosis of melanoma patients with unflattering images of McCain’s cancer scars. “The relevance of knowing the details of his course with melanoma are very important,” says Dr. Michael Frakin, a palliative care specialist from Eureka, Calif., in the ad. “Another bout of cancer for John McCain while he is president of the United States would profoundly impact his capacity to lead.” Another doctor outlines the severity of melanoma. “Melanoma is the deadliest of skin cancers and the chances of survival, if you have melanoma spread through your body, are very, very slim,” warns Dr. Noah Craft, a melanoma specialist from Los Angeles. The ad concludes with text on screen asking “Why won’t John McCain release his Medical Records?”

According to Democracy for America communications director Daniel Medress, the groups are pushing for McCain to release his medical records more openly and exhaustively than his campaign did in May. Medress reiterated what some in the media complained about at the time — that not enough reporters were given ample time to review the documents. “Because of all the very real medical risks that are present with John McCain, a free vibrant, press needs to be able to report these matters,” Medress said.

The McCain campaign has a policy of not commenting on third-party ads, and a spokesperson from the Republican National Committee could not be reached for comment.

The ad’s release comes fresh on the heels of a new poll that suggests McCain’s age, more so than Barack Obama’s race or Sarah Palin’s gender, could be a factor with voters. Thirty-six percent of respondents to a Harris poll released today said it would be a bad thing if someone older than 70 years were elected president — three times the number who said the same of a qualified black or female candidate.

Along with the ad, the groups have circulated a letter that more than 2,300 doctors from around the country have signed, urging McCain to offer “full, public disclosure of all his medical records.” None of the doctors have specifically examined McCain as a patient, but according to Medress, that, in “many ways, is the point.”

“This is an incredibly important issue that’s not being discussed,” Medress said. “The folks who are featured in the ad and signed on to the letter are experts in this. This is not some sort of partisan hitch-job.”