WILLIAM BRANGHAM: For years, the influential Koch network, the conservative political organization founded by billionaires Charles and David Koch, has held a fund-raiser in Palm Springs.

It attracts wealthy conservative donors from around the country.

In 2018, Justice Clarence Thomas attended this fund-raiser, one of several he allegedly attended over the years.

That's according to another new investigation by ProPublica.

Like their previous reports, this raises serious questions over Justice Thomas' undisclosed connections to powerful people who have brought cases before the Supreme Court, including one of the most watched cases in the court's upcoming term.

We are joined now by one of the reporters who helped break this story, ProPublica's Joshua Kaplan.

Joshua, very good to have you back on the "NewsHour."

Before we get to Thomas and his role here, can you just tell us a little bit more about the Koch network and its aims?

JOSHUA KAPLAN, ProPublica: Yes.

So, the Koch network was founded by billionaire industrialists Charles and David Koch to push America toward their libertarian vision for the country.

And it's one of the most powerful and influential political groups of the last half-century.

So, to name a couple things, they helped cultivate the Tea Party movement in the Obama years.

They're credited for helping stamp out GOP support for combating climate change.

And behind that influence is a huge fund-raising operation.

And the marquee fund-raising event is this annual donor summit that they have out in Palm Springs, which is what Thomas attended.

To score an invite to that event, donors have to give at least $100,000 a year, and many give a lot more than that.

WILLIAM BRANGHAM: So you report in your new investigation that Justice Thomas flew on a private jet to this fund-raiser.

He didn't disclose that.

We don't know who paid for that jet.

But beyond that, what else is the conflict here for the justice?

Are there members of the Koch network who had business before the court?

JOSHUA KAPLAN: Yes, so they have had multiple cases at the Supreme Court.

It's a pretty extraordinary position he's been in, having served as a fund-raising draw for this network.

I mean, and it's also judges sit in a unique position of public trust.

They -- the judiciary has very lofty ethics rules to govern -- to protect their independence and impartiality.

You have -- there's -- political activity is banned, for instance.

So is participation in fund-raising.

We talked to one former federal judge appointed by George Bush, who told us that he couldn't imagine Thomas would think it was appropriate to go to the Koch donor summit, that it took his breath away.

He said that, if he had done the same thing when he was a lower court judge, he would have gotten a letter starting a disciplinary process.

But those ethics rules only apply to the lower courts.

At the Supreme Court, the justices decided what's appropriate for themselves.

WILLIAM BRANGHAM: There was a spokesperson for Koch's group, which is known as Stand Together.

They were cited in your report, and they sent us a similar statement.

And their report -- their statement said, in part - - quote -- "There is a long tradition of public officials, including Supreme Court justices, sharing their experiences, ideas and judicial philosophy with members of the public at dinners and other events.

Our events are no different."

How accurate is that?

Is there a distinction to be made here?

JOSHUA KAPLAN: Yes, I mean, I think there is a distinction to be made.

This is a political organization.

And it is also -- I mean, like I said, I mean, this -- to other judges we talked to, this was shocking.

And, yes, as they said, many justices and judges give speeches, they go on book tours.

But I'm not sure -- I mean, certainly, the experts we talked to, the judges we talked to, the judiciary's rules itself consider that very different than attending an event for political donors.

WILLIAM BRANGHAM: We heard today that - - again, similar calls coming out of Congress that your reporting has further sort of incensed the Democratic Party, in particular, saying that there needs to be any reform.

Do you have any sense that the Supreme Court will change its ethics guidelines or that Congress or the Senate will do anything about this?

JOSHUA KAPLAN: I mean, for the Supreme Court, it's hard to say from the outside.

But John Roberts and Brett Kavanaugh have both said that they hope the court will take concrete steps at some point in the near future.

I mean, its -- members of the court had been publicly saying that the court will adopt a code of ethics for four or five years now, at least.

It hasn't happened yet.

But -- and I think there's potentially challenges with, do they need unanimity?

And how does one get that consensus?

In terms of Congress, there's a bill that has been advanced out of the Senate Judiciary Committee that would require the Supreme Court to adopt a code of conduct and also implement some other -- a couple of other ethics rules that so far is not going nowhere.

I believe the majority -- Senator Schumer has said that he thinks it would be filibustered, and so he's not advancing it to the floor.

WILLIAM BRANGHAM: All right, Joshua Kaplan of ProPublica, thank you so much for being here.

JOSHUA KAPLAN: Thanks so much for having me.