With Republican members of Congress becoming increasingly anxious about the unpredictable if not reckless president, the question of how Donald Trump could be removed from office has become more prevalent on Capitol Hill.

The thinking used to be that the 2018 midterm elections might go a long way toward deciding whether the next Congress would take up this question. But with all the president’s recent saber-rattling, combined with his impulsiveness and alarming tendency to ignore his most qualified advisers, the matter is now considered more urgent.

As is well known, the Constitution sets forth two ways to remove a president. Article 25, which establishes the mechanism for getting him out of office if he’s seen as “unable to discharge the powers and duties of his office,” is clumsy and perhaps unenforceable. Then, of course, there’s impeachment. Observers have insisted that Republican leadership would never allow an impeachment proceeding against Trump, or that too many Republicans fear “the base” to move against him. Such things can change over time. Richard Nixon had a base until he didn’t. Yes, Trump’s might be different. I’m not, however, addressing the question of the likelihood of such a proceeding but, rather, if Congress were to seriously take up the matter of impeachment, what might be valid charges against him. Depending on what special counsel Robert Mueller’s investigation turns up, there could be several such charges.

To impeach a president (in the House, by a majority) and then convict him (in the Senate, by a two-thirds vote) is appropriately difficult. It shouldn’t be easy. But it’s a critically important instrument for holding a president accountable for his actions. Impeachable offenses aren’t the same thing as crimes listed in the U.S. Code. In fact, that’s the point: A president can be held accountable for actions that aren’t necessarily crimes. A crime might be an impeachable offense—but not all impeachable offenses are crimes.

An impeachable offense is a crime against the Constitution, or the body politic—as Alexander Hamilton said, “injuries done immediately to the society itself.” Impeachment isn’t a process by which an established set of principles is enforced. There’s no tablet to be taken down from on high and followed; there’s no code of offenses for which a president can be charged. There are precedents, but they’re not binding, which is a good thing. Two of the previous impeachments, of Presidents Andrew Johnson and Bill Clinton, were essentially partisan exercises that fell short of the needed Senate votes. On the other hand, Richard Nixon would have been impeached and convicted had he not given up the presidency to avoid that fate (and to hold on to his pension). The proceeding against Nixon in effect succeeded because it was serious and bipartisan. Whether such a formation is possible in today’s politics is questionable, but as the Republicans get increasingly worried about Trump, a bipartisan move against him becomes less far-fetched than has been widely assumed.

Having sat through weeks of deliberation by the House Judiciary Committee when it considered the impeachment of Nixon, and having kept up with the subject ever since, including talking to constitutional scholars, I believe that there are valid charges to be brought.

A critical principle adopted in Nixon’s in-effect impeachment that could also be applied to Trump was, to my mind, the most significant adaptation of the impeachment power by the House Judiciary Committee. Article II listed a number of “abuses of power” committed by Nixon himself or through his subordinates—including wiretapping, use of the IRS against Nixon’s “enemies;” the break-ins of the Democratic National Committee headquarters at the Watergate and, far more menacing, into the office of the psychiatrist of Daniel Ellsberg, who had leaked the Pentagon Papers. The drafters were interested in looking for a “pattern or practice” of a certain kind of behavior.

Thus the drafters of Article II significantly broadened the scope of impeachable offenses by holding the president accountable for the acts of his subordinates. Under this principle, winking and nodding but not saying anything incriminating (or caught on tape), wouldn’t exempt a president from responsibility for their acts. Thus the answer to the persistent question—Did Nixon know ahead of time about the Watergate break-in?—is, “It doesn’t matter.” He gave enough signals to his subordinates that he wanted to “get the goods” on Lawrence O’Brien, the then-chairman of the DNC.

Thus, even if Trump’s hand isn’t found in any proven collusion by his campaign aides or associates with associates of Vladimir Putin, or even Putin himself, Trump himself could be impeached for collusion.

The charge would be that through his aides and associates, he aided and abetted a foreign adversary in subverting the American democratic electoral system set forth in the Constitution. Of course if Trump himself is proven to have colluded with the Russians to tilt the election in his favor—whether or not these efforts can be shown to have accounted for his victory—he could be impeached and convicted for those same infringements of the Constitution. Though such activities would have occurred before Trump took office, it’s argued that working with foreign interests to bring about one’s own election would be an offense against the polity and the constitutionally prescribed democratic election process and could be considered treason. (Treason, as it happens, is one of the few specific impeachable offenses listed in the Constitution.)

All of this, of course, depends on what special counsel Robert Mueller finds. Mueller is known to be particularly interested in a meeting held in Trump Tower in June of 2016, in which all of the top campaign officials sat down with a Russian lawyer known to be close to the Kremlin who had elicited Donald Trump Jr.’s interest by saying that she had dirt on Hillary Clinton. The president himself got involved in writing a false statement for his son, Donald Jr., to give to the press about that meeting.

While no one offense may be sufficient to impeach Trump, and perhaps shouldn’t be, a combination of potential offenses would be harder for Congress to ignore, should there be an impeachment proceeding.

Then there are the possible Articles of Impeachment based on what we know now:

Obstruction of Justice.

This charge has precedent, particularly in Nixon’s case (though it was also brought against Clinton for lying to a grand jury about his sexual relationship with an intern). And there are sufficient examples of obstruction or attempts thereof on Trump’s part to support it as a charge against him. There are several examples of Trump’s attempting to obstruct an investigation into possible collusion by him or his campaign and associates with the Russians in their effort to tilt the 2016 election. They would include: Trump’s demanding of “loyalty” from the previous FBI director, James Comey, whose continuation in the job depended on Trump’s approval; Trump’s asking Comey to go easy on investigating Michael Flynn, his initial national security adviser. Then there were Trump’s calls to former heads of intelligence agencies asking them to try to convince Comey to halt his investigation into the Trump campaign’s possible collusion with Russia; and Trump’s firing of Comey. Lest there be any ambiguity (which the White House tried to sow) about Trump’s motive, there was his confessing to Lester Holt of NBC News that when he fired Comey, he had on his mind “this Russia thing, ” and the following day, Trump’s telling high Russian officials in the Oval Office that firing Comey had relieved “great pressure” on him. Those acts make a strong basis for charging Trump for the crime of obstruction, which requires a showing of intent, as well as an impeachable offense.

Accepting Foreign Emoluments.

The framers of the Constitution were so concerned that the country’s officials—including its ambassadors stationed abroad as well as the president—could be swayed by foreign governments and potentates that they wrote into Article I of the Constitution a prohibition on any “person holding any office of profit or trust under them,” accepting any “present, emolument, office, or title, of any kind whatever, from any king, prince, or foreign state.” This “Emoluments Clause” has never been tested, but some analysts believe that it could apply to payments that Trump’s properties around the world receive from foreign officials. Particular attention has been paid to Trump’s opportunity to receive profits from foreign governments making use of his swank hotel a short walk from the White House.

Trump has said he’d donate any profits from his high-priced D.C. hotel to charity, but no plan for this has ever been made public. His daughter Ivanka has said she would handle any conflicts that might arise from the president’s ownership position in the hotel, which doesn’t exactly put the matter in neutral hands. A suit concerning emoluments has been brought by the watchdog group Citizens for Responsibility and Ethics in Washington (CREW), which argues that the clause prohibits Trump-owned businesses from accepting any payments from foreign governments. This issue is likely to come up in any congressional consideration of impeaching Trump.

Abuse of the Ethics Rules.

This, too, would be a new kind of impeachable offense, but it’s one rendered conceivable since Trump made the major problematic decision to not relinquish his ownership positions in the family businesses. The public was supposed to be reassured that Trump’s pledge that he would turn the management of the large family concern over to his sons and he wouldn’t talk to them about the matter. How such a restraint was to be enforced was left unexplained. The offense to the body politic is that the president could be helping his businesses profit from his position at the expense of government policy. This could extend to his arranging for his children to benefit from his exalted foreign contacts, for instance in allowing daughter Ivanka, who in addition to holding a stake in the family company owns clothing and jewelry businesses, to sit in on his meeting with the Japanese Prime Minister early in the administration. (Like Trump, Ivanka and husband Jared Kushner, both on the White House staff though they take no salaries, continue to receive income from their companies.)

Is the Trump family to be enriched by the presidency of the patriarch? Is the public simply to trust that the family keeps its business and government matters separate and that the sons don’t talk to the father about them? Why should the Trump family have advantages over private citizens? The impeachable offense could be that Trump has abused power by using his position as president for his own and his family’s enrichment and risked that government policies could be affected by his private interests. Questions have also been raised about Trump’s friendliness toward dictators of countries where he owns hotels, including Turkey and the Philippines. It’s recently been learned that Trump was seeking to build a large Trump Tower in Moscow while he was running for president (efforts to win Russian approval of this project had begun earlier and was abandoned early in the campaign for lack of permits and land).

And then there’s Trump’s famous soft spot for Vladimir Putin. Trump often claimed that he didn’t have business in Russia but that wasn’t for lack of trying. In May, of this year Trump’s tax lawyers sent a letter to the Senate Judiciary Committee saying Trump had not received any income from Russian sources over the past 10 years “with a few exceptions.” The conservative foreign-policy analyst Max Boot has written, “Trump has sought and received funding from Russian investors for his business ventures, especially after most American banks stopped lending to him following his multiple bankruptcies. Eric Trump said in 2014, “We don’t rely on American banks…. We have all the funding we need out of Russia.”

Lying to the Public.

This is a new suggestion. While, of course, past presidents have lied in office, it’s possible that a persistent pattern of lying to the American people by the president and his staff, often at his direction, could justify an impeachable offense. The rationale for such a charge is that at some point a difference of degree becomes a substantive difference, and Trump’s obsessive lying has become substantively different from anything that has gone before. How can a democracy function or a president be accountable if the public is pummeled with numerous lies on a daily basis?

The Founders struggled to find ways to make sure that the president didn’t become a king, and the way to do that was to tie him in a web of attachments and obligations to the congress and the citizenry, to assure, by various means, that the president can be held accountable. The accountability of a president is critical to the democratic system, and impeachment is the Constitution’s provision for holding a president accountable, failing all else.

Refusal to Release Tax Returns as President.

Trump’s adamant refusal to release his returns reverses the practice of his predecessors going back over 40 years. (Richard Nixon, of all people, was the first president to release his tax returns and until Trump his successors all followed suit.) While releasing one’s tax returns as a candidate for the presidency is a voluntary act, as is a president’s doing so, a major principle of the U.S. government is that public officials are accountable to the public, and transparency about their private holdings is a crucial element of this. This is why nominees for many government positions below the presidency must submit agonizingly detailed accounts of their private holdings, including their tax returns for several years. People who vie to be on a presidential ticket are usually asked to submit their tax returns. As candidate for vice president, Mike Pence made public 10 years of tax returns (which showed modest incomes). Presumably, Trump is withholding his tax returns because they contain something he doesn’t want publicly known. Trump may yet be asked to surrender his tax returns to the special counsel. What will he do then?

This charge, like some others, may not stand on its own in impeachment proceedings, but could be part of a combination of charges that Trump puts his private interest over that of the public.

A Report by the Special Counsel to Congress That the President Has Committed Crimes.

There’s been a long debate over whether a sitting president can be indicted, but it’s been generally concluded that a sitting president cannot be subjected to a criminal trial. That leaves impeachment as the remedy. On the other hand, a former president can be tried, which was what led to a major ruckus when Gerald Ford pardoned Nixon after he’d left office. Therefore, if the special counsel finds criminal violations on Trump’s part they would be reported to the Congress, which could settle the matter through the impeachment process.

Thus, while Trump proceeds with his turbulent presidency, and his polls go up and down—they’re awfully low for what are still his early months in office—he’s skating on ever-thinner ice.