Here’s something that will really test whether Democrats can successfully use the Medicaid expansion as a weapon against Republicans in a deep red state where the overall health law remains unpopular.

The Tea Party group Americans for Prosperity, which is backed by the billionaire Koch brothers and is spending huge money on multiple Senate races, has announced a major campaign in Louisiana to pressure state lawmakers not to agree to expand the state’s Medicaid program under Obamacare, as Dems want to do.

Senator Mary Landrieu — a top target of Republicans this cycle — has embrace the expansion, even as she has called for fixes to the law overall. Her opponent, GOP Rep. Bill Cassidy, opposes it.

The battle over the expansion was already set to heat up when Louisiana’s legislative session begins in March. Dems in the state think AFP’s entrance into the fray will allow them to elevate the fight in a way that will help Landrieu, by framing Cassidy’s opposition to it in a more negative light, as a key piece of a national Tea Party agenda associated with the Koch brothers.

Right now Dems are highlighting AFP’s opposition to Landrieu’s flood insurance bill, a huge issue in the state, and tying it to the Kochs. As Alex Roarty smartly notes, this shows AFP’s deep-pocketed support is not risk free for Republicans. This dynamic could appear on the Medicaid expansion: Dems say they’ll highlight AFP’s lobbying on the issue to again draw attention to the outside cash helping Landrieu’s opponent.

You might be surprised to hear that Dems are willing to pick a fight over a part of Obamacare. After all, pundits keep telling us the law is nothing but a liability for Dems, and a certain winner for Republicans, right?

In the real world, the politics of Obamacare are more complicated than that. Landrieu is one of the most vulnerable Dems of the cycle, and she has touted efforts to fix the law. This sort of thing is widely portrayed as running away from Obamacare. Yet Landrieu has also attacked Republicans aggressively over their opposition to the Medicaid expansion, launching a petition calling on Governor Bobby Jindal to embrace it. While Obama’s unpopularity is certainly a liability, Landrieu tries to turn that on Republicans, arguing that their dislike of Obama prevents them from doing the right thing for hundreds of thousands of Louisianians:

Our governor may not like the president, but this is not about the president. It’s about providing health coverage for 240,000 Louisianians who work 40 or 50 hours a week, but still make too little to qualify for assistance in the new marketplace — and too much to qualify for Louisiana’s current Medicaid.

The politics of the Medicaid expansion have taken on a kind of life of their own, separate from Obamacare overall. It has allowed red state Dems to embrace parts of the law while implicitly hitting Republicans over their ideological fixation on full repeal, which would take health coverage away from millions. These Dems don’t talk about Obamacare, obviously. But they stand up for the core goal of expanding coverage to those who lack it (as Michelle Nunn has done by calling for the expansion in Georgia), and criticize Republicans for wanting to take it away from folks (as Alison Lundergan Grimes has done in Kentucky, where the expansion is in full force).

The ability of Dems to turn out core groups like minorities and lower income women – single women and waitress moms — will be pivotal to Dem hopes to holding the Senate. As the nonpartisan analyst Jennifer Duffy has pointed out, while turnout tends to lag among these groups in midterm elections, the Medicaid expansion could help mobilize them. In this context, the coming battle over the expansion in Louisiana will be worth watching as a key tell.

Greg Sargent writes The Plum Line blog, a reported opinion blog with a liberal slant -- what you might call “opinionated reporting” from the left.