One of the Democratic presidential candidates' leading ideas to lower drug prices would come with intense legal battles and thus an uncertain fate.

Why it matters: Democrats' plans to seize drugmakers' patents could give patients some relief from high drug costs, if they work, but they'd be testing the boundaries of the law in order to get there.

What they're saying: Sens. Kamala Harris, Elizabeth Warren and Bernie Sanders have said they'd use "march-in rights" to take away the patents on expensive drugs. South Bend, Ind. Mayor Pete Buttigieg has also said he'd rely on similar tools.

How it works: "March-in rights" come from a decades-old, bipartisan law that allows the government to award a generic competitor the rights to make and sell a patented drug that was developed using public funding, in certain circumstances.

They've never been used before.

A second method, referred to as "Section 1498," applies to every patent regardless of funding. It's been used in the past, but not recently for prescription drugs.

Yes, but: There's a formal process to challenge individual uses of the authority, and it would also be vulnerable to challenges about whether it should exist at all in this context.

The biggest question regarding march-in rights is whether a drug's price can be used to trigger them.

"Some have argued that the law was not intended to be used for high-priced drugs, and that the law isn't available for use on high-price drugs, but that hasn't been tested in court," Washington University law professor Rachel Sachs said.

Section 1498 may be easier to use than march-in rights, Sachs said, but could still be challenged.

The other side: The use of march-in rights "will only jeopardize our country's ability to deliver new medicines to address our most costly and challenging diseases," said Tom Wilbur, a PhRMA spokesperson.