Senate Judiciary Committee Chairman Lindsey Graham, R-S.C., may rue the day he sided with Donald Trump Jr. against a valid subpoena by the Senate Intelligence Committee.

Graham’s advice that Trump Jr. refuse to testify was way out of line in principle, and could prove harmful to his own committee’s work in practice.

Graham at first said that Trump Jr. should refuse even to appear before the Intelligence Committee. At least he later amended his advice. Now he says he meant the presidential son should comply technically with the subpoena by showing up at an Intelligence Committee hearing, but then refuse to answer any question by pleading the Fifth Amendment.

The original statement was worse, indeed awful, but even the modified advice is wrongheaded.

A subpoena from Congress, like one from a court, is a binding legal summons. The only valid reason for refusing to comply is if the subpoena demands information or material that is unlawful for the subpoenaed person to supply. Mere disagreements over politics or mere inconveniences (“I’ve already answered some of their questions!”) do not qualify as valid excuses. Graham himself made that very point back in 1998 when arguing in favor of impeaching then-president Bill Clinton.

Congressional oversight — over the executive branch, over law enforcement, and over U.S. intelligence gathering — is an essential part of constitutional government. Courts repeatedly have recognized the oversight power as necessary and vast. Indeed, oversight by the elected representatives of the people is an essential safeguard of liberty.

Graham knows this. Moreover, in very practical terms, he should appreciate the oversight and subpoena authority even more than most. The Judiciary Committee that Graham leads is, historically, one of the committees that needs and uses subpoena power the most. By openly saying a citizen should ignore or undermine a subpoena, he weakens his own standing, at least morally and perhaps at the margins legally, if he as chairman wants to issue subpoenas in the future.

Graham also undermines Senate traditions and comity by so disrespecting a co-equal committee chairman as he now has done to Intelligence Committee Chairman Richard Burr, R-N.C. The insult is especially pronounced when inflicted upon a chairman of one’s own party, and in this case from a neighboring state as well.

The damage was mitigated, but only slightly, when Graham clarified that he thinks Trump Jr. should comply but “take the Fifth.” While paying belated homage to the importance that Congress’ legal powers of subpoena be preserved, Graham still is advising a key witness to withhold information another committee and its chairman think vital to the national interest. As the committee in question here is Intelligence, with the national-security implications involved and often with the added element of secrecy, Graham in theory could be undermining a particularly important inquiry whose importance he does not fully understand.

Graham’s only stated reason for doing so is that he thinks it’s important for the investigation into Russian perfidy to be “over.” Well, who is he to say so? The criminal investigation may largely have run its course, but that doesn’t mean there aren’t loose ends to tie up regarding the crucial, underlying reason for the intelligence investigation in the first place: learning how the Russians went about trying to subvert U.S. elections.

To advise someone to “take the Fifth” implies both that that person remains subject to criminal liability (which probably is not the case here) and that the person’s knowledge is immaterial to Congress’ due interests in protecting American security. Both assumptions likely are wrong.

Graham’s advice is thus ill-considered, and potentially dangerous.