RACHEL MARTIN, HOST:

President Donald Trump has been pretty clear about the fact that he has no intention of releasing his tax returns, as every other modern president before him has. That's a problem for Congressman Mark Sanford of South Carolina. He has co-signed legislation - proposed legislation - that would require future presidents-elect to release their tax returns. Congressman Sanford joins us now. Thanks so much for being with us.

MARK SANFORD: My pleasure.

MARTIN: You have really taken this on, beating this drum in a way that not many other Republicans have. Why is this so important to you?

SANFORD: It's important on a couple of different levels. This is a 50-year tradition. And so to let this die off is to let a piece of transparency die off that I think is vital to every voter out there, whether Republican, Democrat or independent in saying, is this the person that I want to vote for? It's equally important, though, down-ballot. This ultimately is not about Donald Trump's tax returns. And my point has been, look, if the president stopped releasing their tax returns, guaranteed - folks down-ballot will not release theirs. And, again, I think that's taking a piece of information that's vital to voters deciding, whether at the federal or state level.

MARTIN: Why haven't more of your Republican colleagues gotten on board with this issue?

SANFORD: Because nobody wants a bad tweet from Donald Trump. I mean, I get it. But, again...

MARTIN: You don't care about that?

SANFORD: ...This isn't about him. This isn't about him. It's about sustaining certain traditions that have served our republic awfully well.

MARTIN: You don't care if you get a bad tweet from President Trump?

SANFORD: No, I care. I mean, I don't want that. Nobody's looking for a bad day. The president can be quite vigorous in his pronouncements. And you don't want to be at the receiving end of one of those. But I think that, in this case, not letting a 50-year tradition die off or fighting if it is doing so is of greater importance.

MARTIN: Last week, the House and Senate overwhelmingly supported bills that would impose sanctions on Russia. And, notably, this legislation contained provisions that curtailed the president's power to change those very sanctions. Do you see this as a sign that your party might be getting more comfortable with opposing the president?

SANFORD: Yeah. There is a honeymoon period to politics. And nobody was going to walk particularly far out on the plank. Or very few people would walk very far out on the plank in opposing a new, incoming president. He's done some weird stuff. It's been noted. It's impacted, I think, some of his level of respect with regard to the country at large or certain members of the political process in Washington. And I think people are feeling a little bit freer to say, respectfully, we disagree here. And, therefore, we're going to do this.

And I think that's a good thing. He may do more to reinvigorate separation of powers than anybody has done in recent history. And if so, I think that'd be a very good thing because the idea of a vigorous Congress and a vigorous executive branch and a vigorous judicial branch, each being checks upon the other, is what the Founding Fathers intended. And what has happened in recent history is, oftentimes, the legislative branch become sort of the lapdog to the executive branch. And that's not what the Founding Fathers envisioned.

MARTIN: Congressman Mark Sanford from the state of South Carolina - we've been talking about a bill that would require future presidents-elect to release their tax returns. Thank you so much, congressman.

SANFORD: My pleasure. Take care.

(SOUNDBITE OF MICHITA'S "NANDODEMO (INSTRUMENTAL)") Transcript provided by NPR, Copyright NPR.