From this New York Times deep-dive:

Other candidates rose and faded in the process: Senator Tammy Duckworth of Illinois powerfully impressed Mr. Biden’s search team, but his lawyers feared she would face challenges to her eligibility because she was born overseas.

…

Ms. Duckworth was regarded by Biden advisers as among the candidates likeliest to help him achieve a smashing electoral victory in November. But legal advisers to the campaign expressed urgent concern that Ms. Duckworth could face challenges to her nomination in court: She was born overseas, to an American father and a Thai mother. While Mr. Biden’s team believed Ms. Duckworth was eligible for national office, campaign lawyers feared that it would take just one partisan judge in one swing state to throw the whole Democratic ticket off the ballot.

I don’t have strong thoughts on the merits of whether Ms. Duckworth is a “natural born citizen.” And even if 2020 was relatively quiet on natural born citizen challenges (until recently!), it’s interesting to see the political process play out here.

I’ve highlighted that there are several bodies that can ascertain candidate eligibility outside the judiciary: the voters, presidential electors, and Congress. Political parties are a good addition. But perhaps it’s worth adding a category for vice presidential candidates, as the nominee and his team can make judgment calls about eligibility, too.

But, disappointingly, the campaign lawyers made a pessimistic call. My work strongly resists the call for other bodies—particularly judges—to review candidate eligibility. Indeed, I’ve written extensively about that. I think many times there are not rules in place to even allow courts to review a candidate’s qualifications. A declaration from Congress could go a long way to ensure that a candidate’s eligibility is not questioned elsewhere. I think most states don’t have statutes in place to authorize review of candidate qualifications, and, indeed, I think they shouldn’t. And there’s always appellate review—granted, one challenge of late-breaking litigation (like naming a vice presidential candidate in August when ballot printing begins in a matter of weeks) increases risk and uncertainty.

Furthermore, it’s also a fundamental weakness of the National Popular Vote. Could one state exclude a candidate from the ballot? And if so, doesn’t that throw off the “national” popular vote total? Absolutely.

In short, it’s a highly cautious, litigation-avoidance strategy that kept (at least in part) Ms. Duckworth off the ballot. It’s worth considering whether a veepstakes that took place earlier could have successfully resisted litigation that might have challenged her eligiblity.