Most white Southern Democrats have always been on the conservative side. That’s why, over the years, it’s been so easy for them to slip off the reservation and leave the party.

U.S. Sen. Strom Thurmond, of South Carolina, was probably the most prominent of the early party switchers, becoming a Republican in 1964. Texas Gov. John Connally, a protégé of President Lyndon Johnson, flipped to the Republican side in the early 1970s.

U.S. Sen. Richard Shelby, of Alabama, and Louisiana’s U.S. Rep. Billy Tauzin changed parties after Republicans won congressional majorities in 1994.

At the state level, former Gov. Mike Foster, state Treasurer John Kennedy and state Senate President John Alario are just a few of those who switched parties.

Meanwhile, there are a lot of registered Louisiana Democrats who never bothered to officially change their party affiliation but who consistently vote for Republicans in local, state and federal elections.

There’s another side to that coin, though, as the recent campaign spot for Sen. Mary Landrieu featuring former state Republican bigwig Boysie Bollinger shows.

“I’m with Mary,” Bollinger, a former finance chairman for the state GOP, proudly says in the commercial.

While the Democrat has been in tight races every time she’s run for her Senate seat, she has often counted on the backing of prominent Republicans who are not afraid to be upfront in their support.

Jefferson Parish Sheriff Newell Normand, St. Tammany Parish Sheriff Jack Strain and former Gov. David Treen are among the Republicans who backed Landrieu in previous campaigns.

The Wall Street Journal recently noted the support Landrieu picked up from big businesses that are sending money to her campaign along with their love. Political action committees for ExxonMobil, Chevron, ConocoPhilips and even the hated BP have contributed to her.

With Landrieu recently taking the chair of the Senate Energy and Natural Resources Committee, those companies likely will have business before her panel. So will energy-industry trade groups, which also contributed via their political-action organizations, according to the Journal.

All of this has those Louisiana Democrats who aren’t conservative — and who may be more interested in environmental and climate issues — rolling their eyes. In their minds, she’s just another Democrat who should have switched parties, at least when it comes to taking on Big Energy, given that she even backs building the Keystone pipeline.

But while they’re rolling their eyes, they’re also going to have to decide whether they’ll hold their noses and vote for Landrieu anyway.

They may not see a dime’s worth of difference if Landrieu’s Senate seat is held by her or by a Republican. But there’s a larger picture: Would it make a difference to them if the Senate were controlled by a Republican majority? That could happen if Landrieu loses.

At least left-of-center Democrats see a reliable vote in Landrieu on such issues as national health care or unemployment insurance. And they know she is likely to be just as reliable a supporter of President Barack Obama’s federal judgeship candidates, including possibly another U.S. Supreme Court pick.

There’s already a built-in enthusiasm gap in mid-term elections that usually favors Republicans. Landrieu’s task this year is to narrow the gap by bringing traditional Democratic voters home — that is, to the polls.

She’ll have to gin up serious enthusiasm among voters who aren’t crazy about her positions. It might take the specter of a Republican-controlled Senate to do that, or the possibility that if she loses, her replacement will be beholden to the Koch brothers, the bête noire of modern-day liberals.

Dennis Persica’s email address is