If it ain’t broke, don’t fix it.

That may be as good a reason as any for the U.S. Senate campaign of Republican U.S. Rep. Bill Cassidy to continue to harp on its near-obsessive theme: that the Democratic incumbent, Mary Landrieu, marches in lockstep with President Barack Obama. The “he/she’s with Obama” line was trotted out by Republican Senate candidates across the country as they successfully capitalized on the president’s unpopularity to unseat other Democrats and gain control of the Senate.

In the run-up to the general election Tuesday, Cassidy repeated the number “97” so many times — as in, Landrieu has voted in support of Obama’s legislative agenda 97 percent of the time — that he may start to think that’s his age and wonder what happened to his Social Security checks (he’s actually 57).

Cassidy took 41 percent of the vote, a close second to Landrieu, but with both short of the majority needed for an outright victory Tuesday, they will meet in a head-to-head runoff Dec. 6. With another Republican, Rob Maness, taking 14 percent of the primary vote while running a tea-party campaign to Cassidy’s right, the assumption is that Cassidy, who is plenty conservative himself, and certainly more so than Landrieu, will pick up the lion’s share of Maness’ support and cruise to victory in the runoff. Several political obituaries of Landrieu have appeared already — although, given her past record of coming back from the dead, they may be premature.

Cassidy has kept up the Obama drumbeat in recent days: In announcing his agreement to appear in one televised debate with Landrieu, he countered her suggestion for six debates with his own proposal for — wait for it — 97. He tossed in a couple of other issues, challenging her on Medicaid cuts, “Obamacare” and the failure to win approval of the Keystone pipeline, but he went back to the well when he said he’d debate her as many times as she “barnstorms the state” with the president at her side.

All of which is lamentable in a way, as Cassidy has a personal story to tell that should appeal to a wide range of voters, and not just angry Republican ones.

Before he entered politics by winning election to the state Senate in 2006, Cassidy practiced medicine in Baton Rouge, specializing in treating diseases of the liver at the Earl K. Long charity hospital. He’s also taught — and continues to teach, part time — at the LSU medical school.

But there’s more, as his Wikipedia entry explains:

“In 1998, Cassidy helped found the Greater Baton Rouge Community Clinic to provide uninsured residents of the greater Baton Rouge area with access to free health care. The Clinic provides low-income families free dental, medical, mental health and vision care through an innovative ‘virtual’ approach that partners needy patients with doctors who provide care free of charge.

“Cassidy has also been involved in setting up the non-profit Health Centers in Schools, which provides Hepatitis B and flu vaccinations to children in the East Baton Rouge Parish School System.

“In the wake of Hurricane Katrina, Cassidy led a group of health care volunteers to convert an abandoned Kmart building into an emergency health care facility, providing basic health care to victims of the natural disaster.”

Cassidy has alluded to his medical background in his campaign, both in debates, when he attacks “Obamacare” and the expansion of Medicaid in the state, and in campaign commercials that feature him in a white lab coat and green surgical scrubs wearing a stethoscope. But apart from Cassidy’s Twitter rebuttal to Landrieu on Friday, that’s about it.

Maybe Cassidy is concerned that the more rabid of his supporters will be turned off by anything that makes him look like a bleeding-heart do-gooder. Or maybe he thinks former President George W. Bush forever trashed the “compassionate conservative” brand.

Most likely, his campaign figures that if it ain’t broke, don’t fix it.

Follow Gregory Roberts of The Advocate Washington bureau on Twitter, @GregRobertsDC.