What do you do after you’ve made your very best case and fallen short?

Probably about what Democratic U.S. Sen. Mary Landrieu is doing as she struggles mightily to reshape her re-election contest in its final days, after earning just 42 percent of the primary vote against two GOP opponents in an undeniably Republican year in an undeniably Republican state.

While the runoff is a new race, Landrieu didn’t find herself in a deep hole because she failed to make her point the first time around.

Despite a few distractions, gaffes and sideshows, she did a perfectly good job of explaining that she’s willing to break with her party on oil and gas issues, and that her willingness to horse trade, her seniority and her “clout” have helped Louisiana secure all sorts of disaster aid, not mention a hefty cut of BP fines and future offshore oil revenues. She talked relentlessly of how her new position as chair of the Energy and Natural Resources Committee could benefit the state — an argument that’s now moot, given the voters elsewhere have already handed Republicans the majority. She cast the election as a local, not a national, one.

No, she’s in serious danger of losing her seat after three terms because the majority of voters who turned out on Nov. 4, and white voters in particular, had other priorities — the same sort of priorities that her runoff opponent Bill Cassidy has pushed in a campaign that’s singularly focused on national party politics in general, and on President Barack Obama in particular. And there’s no real reason to think that those who show up again for the runoff will feel differently.

That hasn’t left Landrieu with many good, new options, which would explain why she’s seizing on several bad ones.

Landrieu ran the best campaign ad of the primary season; the folksy testimonial from shipbuilder and Republican powerbroker Boysie Bollinger that “she does big things for Louisiana” was the strongest possible message from the strongest possible messenger. Now that we’re in runoff season, she’s airing the worst.

The ad in question, titled “Whoa,” labels a Cassidy speech from last spring “nearly incoherent,” and shows snippets of the congressman hemming, hawing and tripping over his own words. The idea is clearly to paint Cassidy as unsenatorial, even “weird,” as U.S. Rep. and Landrieu backer Cedric Richmond has called him.

But you know what’s just as unsenatorial? Personal mockery. And you know who else has gotten tongue-tied on occasion? Plenty of people who speak in public, Landrieu included. If anything, this sort of ridicule may make viewers more sympathetic to Cassidy, not more likely to give Landrieu a second look.

The ad’s other big flaw concerns content, or lack thereof. Each time Cassidy is about to get a sentence out, he gets cut off. Kind of makes you wonder whether what he actually had to say wasn’t quite so alarming.

Then there’s Landrieu’s last-minute, lame-duck session gambit to finally secure a Senate vote on the Keystone XL pipeline extension, which she supports but which many environmentalists in her party oppose.

If the idea is to prove that she can deliver (on an issue that’s largely symbolic for Louisiana, since the pipeline would end in neighboring Texas), the gambit also highlights the fact that Landrieu could only get her party’s leaders to go along now that they’re trying to save her neck.

Then there’s the fact that the bill has to go through the House as well, which hands her congressman opponent an opportunity to show that he can deliver, too.

In Landrieu’s defense, there is some strategy behind the ad. Her campaign is betting, correctly, that voters really never got to know Cassidy because he skipped several debates and spent so much of the campaign talking about Democrats, not himself, his record, or his story. There’s strategy behind the Keystone push too, in that it paints her as someone who won’t give up the fight, even when the odds are long.

You work with what you’ve got. Which, despite Landrieu’s three terms and list of accomplishments, isn’t much these days.

Stephanie Grace can be contacted at sgrace@theadvocate.com. Read her blog at http://blogs.theadvocate.com/gracenotes. Follow her on Twitter @stephgracenola.