Richard Painter has yet to become the subject of a Donald Trump tweet. That’s fine with the University of Minnesota law professor.

On the other hand, he did sue the president of the United States last week for violating the Constitution.

Painter is a top ethics scholar who worked as Republican President George W. Bush’s chief ethics lawyer from 2005 to 2007. But he has picked a fight with the GOP’s latest White House resident for refusing to sell off some assets and place the rest into a blind trust, as every president has done for the past four decades. Instead, Trump has turned them over to his sons to manage and promised not to ask about them.

“If Obama had tried that,” Painter said, “we would have impeached him in two weeks.”

So Painter has put his reputation on the line. Through essays, interviews and now a high-profile lawsuit, he continually tells anyone who will listen that Trump’s potential for personal gain or loss from the properties and companies he continues to own creates an unconstitutional conflict with his charge to do what is best for the country.

The issue emerged Jan. 11 when Trump announced that he would not divest some holdings and put the rest in a blind trust as presidents have done for the past 40 years. It bubbled back to the surface Friday when Trump banned travelers from certain predominantly Muslim countries from entering the U.S. but did not extend the ban to any Muslim countries where he does personal business.

“The president’s executive order is offensive and unconstitutional not only because it denies due process and is a thinly disguised ban on Muslims … but it also is heavily tilted toward the predominantly poor countries where the Trump organization is not doing business,” Painter said.

Painter is not bothered by a recent poll showing that a majority of Americans believe Trump has taken sufficient steps to avoid conflicts of interest. Nor is Painter cowed by scholars who question whether he has legal standing to sue the president.

“The average voter will care when a crisis comes up,” Painter said. “If Franklin Roosevelt had had Roosevelt Towers in Berlin and Frankfurt and a $350 million revolving loan with Deutsche Bank in Germany in December 1941, what would he have done about going to war? We just cannot be in that position.”

The White House declined to comment on Painter and referred the Star Tribune to the transcript of the Jan. 11 news conference, where Trump announced his plans not to divest his holdings and said he did not have to if he let his children manage them.

In the New York Times, Trump’s son Eric called the new suit “purely harassment for political gain.”

There are questions about whether Citizens for Responsibility and Ethics in Washington (CREW), which filed last week’s lawsuit and with which Painter is associated, can prove that it has been hurt by Trump’s actions. That is a requirement for standing to sue the president.

In the suit, CREW points to a constitutional ban against the president taking anything of value from foreign governments. The suit says that makes it problematic for foreign government officials to pay to stay at Trump’s hotels or rent space in Trump’s commercial buildings.

For Painter, the suit is part of a career that stretches back decades and has led to dozens of law review articles, books and legal presentations.

“He’s the Number 1 scholar in the country on government ethics,” said Norman Eisen, President Barack Obama’s ethics chief.

Eisen met Painter in 2014 when both were on an ethics panel. But Eisen, whose White House nickname was “Mr. NO,” had been reading Painter’s memos from his years with the Bush administration. Painter seemed like a stickler for avoiding even the slightest appearance of conflicts of interest.

The two hit it off and eventually decided to write collaboratively about what they saw as Trump’s conflict of interest problems. They also joined CREW.

In the 2016 presidential race, Painter first supported Jeb Bush, then caucused for Marco Rubio in Minnesota, then backed John Kasich and finally chose to vote for Hillary Clinton because he was put off by Trump’s comments about Muslims and Mexicans and his threats of a trade war.

Despite charges that Painter’s interests are politically motivated, people who have known him the longest say what he is doing now is just an extension of principles that he’s expressed for years.

Out of Yale Law School, Painter worked on Wall Street during the savings and loan scandal of the 1980s and the quick-buck junk bond era of the 1990s. His published his first article on ethics — on the moral responsibility of corporate lawyers — in 1994. He wrote one of his latest books, “Better Bankers, Better Banks,” with U colleague Claire Hill. Hill called Painter “a joy to work with” because “he is extremely principled and committed to doing what he can to make the world a better place.”

Painter has been teaching professional responsibility to law students for roughly two decades. He, his wife and three children came to Minnesota when he left the Bush White House in the summer of 2007. He says the advice he is giving Trump today is the same advice he gave to people like Goldman Sachs CEO Henry Paulson when he wanted to be treasury secretary under George W. Bush:

Fill out a complete financial disclosure and produce an ethics agreement to sell off assets that suggest any conflict of interest, replace them with cash or conflict-free investments and put everything in a blind trust.

“He’s a plain speaker who works hard to find good ideas and express them in clear language,” said Steve Price, who met Painter as a freshman at Harvard in 1980 and remains close to him. “I’ve never thought of Richard as a liberal. I’ve thought of him as principled.”

“On many issues he’s quite conservative,” Eisen added.

Boston attorney Geoffrey Bok, who went to Harvard with Painter and is one of his best friends, said he is “a little surprised” that Painter has taken on the president in such a high-profile way because Painter “doesn’t seek the limelight.” But once he’s taken a stand, it is not surprising that Painter “would stay in the hot seat.”

Painter criticized Clinton’s refusal to shut down her foundation and her use of a private e-mail server as secretary of state. He also filed a Hatch Act complaint against FBI Director James Comey for disclosing an investigation of Clinton just ahead of the election without revealing that it found no wrongdoing. Through it all, Painter considered Trump’s ethics trouble to be riskier for the country.

“His physical properties — the golf courses, the buildings — all have debt that has not been disclosed,” Painter explained. “We don’t know how much is owed and to whom. People are beholden to their creditors. Do you want a president who is beholden to creditors? Also, commercial real estate may be the next bubble. That’s Donald Trump.”

Win or lose, Painter said he will continue to press the president on ethics issues, especially his conflicts of interest.

Writing with Eisen in Sunday’s New York Times, Painter made good on that promise.

“It appears that immigrants from countries that can afford to do business with the Trump organization are free to come and go from the United States,” they observed. “Immigrants from countries that cannot afford such transactions may very well be detained at the airport and sent home, where some may perish.”

That kind of criticism pushes Painter closer to the wrath of @realDonaldTrump.