By any objective measure, it's a bit of an understatement to say that Senator Don Meredith behaved badly when he began pursuing a teenage girl when she was 16, and eventually had sexual intercourse with her just before and after her 18th birthday.

Though he denies that he encouraged the girl to become intimate with him, the evidence laid out in a report by the Senate Ethics Commissioner, Lyse Ricard, convincingly argues otherwise.

Mr. Meredith, who is married and has children, admitted to some of the liaisons, and responded with a "no comment" when confronted with allegations of other instances of sexual relations brought by the girl, referred to only as "Ms. M" in the report.

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Ms. Ricard has determined that Mr. Meredith failed to "uphold the highest standards of dignity inherent to the position of Senator," and acted "in a way that could reflect adversely on the position of Senator or the institution of the Senate" – two important rules of conduct in the Ethics and Conflict of Interest Code for Senators.

Opinion: A Senate in need of trust must act quickly on Meredith

Ms. Ricard is careful to point out that she is not making a moral judgment about extramarital sex between consenting adults. The private affairs of senators would not normally be in her purview, and nor should they be. It should also be made clear that Mr. Meredith has not been criminally charged, as Ms. M was over the age of consent.

But the Senate Ethics Commissioner found that Mr. Meredith crossed a line when he mixed his role as a senator with his dalliance, creating an imbalance of power between himself and a starstruck teenaged girl who declared herself in love with him.

As a much older man in a position of authority, he should have discouraged her. Instead, he texted Ms. M on his Senate cellphone and met with her in his office. He made promises about putting her on a committee he was creating, wrote her a letter of reference for an internship on Parliament Hill and even raised the possibility of doing business with her parents.

And, according to Ms. Ricard's report, evidence in the form of e-mails, text messages and the girl's testimony indicate that he initiated the sexual contact.

As such, Mr. Meredith acted in a way that reflects poorly on his position as a senator and on the Senate as a whole – a violation, she says, of the Senate code of ethics.

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But here's the catch: The two rules that Mr. Meredith violated only came into effect in June 2016. His contact with the girl began in 2013; had he ended it before June 2016, the Ethics Commissioner might have nothing on him.

The new rules were created in response to the expense scandals that rocked the Senate from late 2012 to 2015. Mike Duffy, Patrick Brazeau, Mac Harb, Pamela Wallin – you remember the names. They were all found by a Senate committee to have claimed illegitimate expenses. Messrs. Harb and Brazeau were charged with criminal offences that were later dropped, while Mr. Duffy was acquitted on several charges in court – including one of bribery for accepting a cheque of more than $90,000 from the chief of staff of then-prime minister Stephen Harper that he used to pay back his expenses.

Under intense public pressure, the Senate suspended Mr. Duffy, Mr. Brazeau and Ms. Wallin for two years. All three are back in the Senate today. Mr. Harb, a former MP, resigned from the Senate in 2013, retiring to a parliamentary pension of $123,000 a year.

The Senate subsequently added its two new rules, in the hope of being able to better discipline members who do not uphold high standards of dignity by, let's say, pretending to live in one place in order to collect living expenses in another, or whose avarice reflects adversely on the position of senator or the institution of the Senate.

The new rules can't be retroactively applied, which makes Mr. Meredith the first senator to have his or her behaviour judged by the standards they impose. And he has clearly violated them. Based on Ms. Ricard's report, and the evidence she has presented, he should resign.

If he refuses to go voluntarily, his fellow members should consider suspending him, or even expelling him – a difficult but not impossible outcome.

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But Mr. Meredith's case is a reminder that senators, unlike MPs, are nearly impossible to remove. They are appointed, not elected; they sit until age 75 – Mr. Meredith has more than 22 years to go – and they are responsible to nobody and to nothing beyond their own conscience. As long as they show up for work and aren't convicted of a crime, they are essentially irremovable.

Were he an elected member of the House of Commons, Mr. Meredith would have to answer to voters. Instead, his fate lies in the hands of his fellow unelected senators. This is the constitutional order we're stuck with, like it or not.