Sanford noted his continued ability to support Trump will “in part be driven by whether Mr. Trump keeps his word that he will release his tax records.”

Sanford notes just two years earlier ― in just one of several instances ― Trump clearly stated he would have no problem releasing his tax returns.

During an interview with an Irish TV station, Trump said, “If I decide to run for office, I’ll produce my tax returns, absolutely and I would love to do that.”

Trump’s bombastic statements, Sanford argues, will be forgotten with time, while the legacy of his actions will linger. 

“Not releasing his tax returns would hurt transparency in our democratic process, and particularly in how voters evaluate the men and women vying to be our leaders,” Sanford wrote. 

Data compiled by the Tax History Project shows that every Republican candidate since Richard Nixon has made their tax returns public. 

Trump has inflated claims over the years about his personal net worth and charitable giving ― two areas that could be cleared up by releasing his tax returns. But said he won’t make the documents public because he’s being audited by the IRS.

While the IRS has said an audit doesn’t prevent Trump from releasing the returns, Sanford, a former governor, was at least sympathetic to how unpleasant the process can be.

“I ran twice for governor of South Carolina, and I released my tax returns both times. To be frank, it felt a bit like a colonoscopy: I didn’t like it, but it was our tradition in South Carolina. The power of staying true to the precedent that had been set prevailed.”

Recent revelations about the finances of Trump campaign chairman Paul Manafort are likely to increase pressure on Trump to release his tax returns. 

The New York Times reported on Sunday that a Ukrainian anti-corruption agency found Manafort’s name on a secret ledger used by the ousted, pro-Russia government of ex-Ukrainian President Viktor Yanukovych. The ledger showed payments totaling $12.7 million designated for Manafort from Yanukovych’s political party. Manafort was a top political adviser to Yanukovych for several years, but it’s not clear if he ever received the payments listed.

The financial and political ties of several Trump campaign aides and donors to entities associated with the Russian government have also raised questions about whether conflicts of interest are influencing the GOP presidential candidate’s views of U.S. policy toward Russia. Trump has suggested he would take a more conciliatory approach toward Russian President Vladimir Putin, whose government has elicited Western sanctions and geopolitical isolation for its takeover of the Ukrainian peninsula of Crimea and support for an armed rebellion in eastern Ukraine. 

An embarrassing hack into the Democratic National Committee and presidential campaign of Hillary Clinton that is widely believed to be the work of the Russian government has also raised suspicions about the Trump campaign’s Russian ties.

Trump himself has denied any investments in Russia or other financial connections to the country. His claims are impossible to verify, however, without access to his tax returns.