SEVERAL POSSIBLE explanations have been offered for President Donald Trump’s strange affection for Russia. He is said to approve of Vladimir Putin’s authoritarian tendencies; to be compromised by embarrassing material in the hands of Russian authorities; to have business interests in Russia. These theories are not mutually exclusive. Yet on November 29th evidence for the last, and perhaps the most plausible, emerged. Deep into Mr Trump’s presidential campaign—indeed several weeks after he had sewn up the Republican nomination—it seems that his advisers were lobbying the Kremlin to help them secure permits and financing to build a Trump Tower in Moscow.

This revelation emerged from an unexpected court appearance by Michael Cohen, Mr Trump’s former personal lawyer. Mr Cohen, who was representing Mr Trump’s interests in Moscow, pleaded guilty in a court in Manhattan to lying to Congress about them—in an effort, he said, to cover up for the president.

Mr Cohen had previously told two congressional committees, which were investigating Russia’s efforts to fix the 2016 election for Mr Trump, that negotiations over the possible Moscow tower had concluded by January 2016, before the onset of the Republican primaries. He also claimed to have discussed the aborted negotiation with Mr Trump on only three occasions. According to court documents filed alongside Mr Cohen’s guilty plea, he had in fact discussed the project with Mr Trump more often. He had also briefed “family members” of the president on it.

He had lied about the timing of the negotiations “to minimise the links between the Moscow project and [Mr Trump]” to “give a false impression that the Moscow project ended before the ‘Iowa caucus’”. This was done, Mr Cohen said in court, in an effort not to contradict the president’s “political messaging” and thereby “to be loyal” to him.

In the drip-feed of Trump-Russia revelations, it is often hard to know what matters. But this is a major development. At the least, it suggests Mr Trump repeatedly lied about his interests in Russia, both before and after the Russian plot to elect him had been uncovered and made public. “I have no dealings with Russia,” he said in January 2017. “I have no deals that could happen in Russia because we’ve stayed away.”

It appears Mr Cohen may not have been the only one of Trump’s advisers to have lied in support of the president on this. Adam Schiff, the incoming Democratic chairman of the House intelligence committee, to which Mr Cohen testified, released a statement saying his guilty plea “highlights concern over another issue—that we believe other witnesses were also untruthful before our committee”. The president’s eldest son, Donald Trump junior, a vice-president of the Trump Organisation, is among the other Trump associates who have testified before it. He told the Senate Judiciary Committee last year that he was only “peripherally aware” of the Moscow deal. (He was interviewed by the House committee behind closed doors.)

Mr Cohen’s plea was also a significant strike for Robert Mueller. Mr Cohen had previously pleaded guilty to several offences, including tax evasion and campaign finance violations, after being prosecuted by the US attorney’s office in the southern district of New York. His latest guilty plea was to a charge brought by the special counsel, with whom Mr Cohen is now co-operating.

Of the dozens of indictments and convictions Mr Mueller has so far served up, this was the first to touch on the president’s business interests—something Mr Trump had attempted to draw “a red line” around. In the process it sheds some light on the possible case Mr Mueller could be building against the president.

He is believed to be investigating Mr Trump and his associates on two related tracks. He is trying to ascertain whether they colluded with the Russian election-hackers—including 25 Russian individuals and three Russian companies—that Mr Mueller has already indicted over the affair. Second, he is investigating whether Mr Trump has sought to obstruct the Justice Department’s investigations into the election-hacking, including by first leaning on and then sacking James Comey, his first FBI director.

It was until recently assumed that the second track against Mr Trump was likelier to bear fruit. Arguably, Mr Trump has been attempting to obstruct justice in plain sight. He said publicly that he sacked Mr Comey over the Russia investigation. He has lambasted the Justice Department and Mr Mueller repeatedly, and last month sacked his attorney-general, Jeff Sessions, in an apparent effort to gain control over them. Yet Mr Trump’s Republican supporters, who would have to desert him for the president to be in any serious impeachment danger, have grown inured to such noisy antics. Republican congressmen refer to his threatened assaults on the rule of law as if to the regrettable eccentricities of an elderly relative. Having defended Mr Trump thus far, it is hard to think of an obstruction case that could convince many Republicans to condemn him.

Apologising for collusion by Mr Trump or his associates with a hostile power’s attack on American democracy would be a harder stretch, however. And Mr Mueller’s investigation does appear to be making accelerating progress on that investigative track. The Moscow tower revelation does not provide evidence of such collusion. But it does more than offer a possible explanation for Mr Trump’s fondness for Russia. It points to an open line of business communication between the Trump team and the Kremlin even as the Russian hacking campaign began. Aaron Blake of the Washington Post notes that the court documents filed in Manhattan suggest Mr Cohen reached a decision to stop pursuing the Moscow tower project “on or about June 14th”. The Post ran a big report on Russian hacking of the Democratic National Committee’s computer network that same day.

Mr Cohen’s plea adds significance to several other Mueller-related developments. It comes only a few days after Mr Trump filed written responses to a long list of questions from Mr Mueller, which are reported to have touched on the proposed Moscow tower. If Mr Trump’s answers differed from the information revealed by Mr Cohen’s plea, the president will have created fresh trouble for himself.

The plea also raises the salience of another strand of Mr Mueller’s collusion investigation. This concerns the alleged efforts of two Trump supporters, political operatives Roger Stone and Jerome Corsi, to encourage or co-ordinate the release of damaging Russia-hacked material by Wikileaks. Tweets by Mr Stone and emails between the two operatives, which were obtained by Mr Mueller, suggest they knew in advance that Wikileaks was about to publish hacked emails belonging to Hillary Clinton’s campaign chairman, John Podesta. Mr Corsi, who this week refused a plea bargain offer from Mr Mueller, has said he came to know this through “divine intervention”.

Mr Mueller’s latest strike also makes it even more desirable that Congress should pass a bill to protect the special counsel from Mr Trump’s threatened attack on him. There is such a bill to hand. Yet the Republican Senate leader, Mitch McConnell, refused once again this week to allow a vote on it. He says it is unnecessary. Yet the president’s attacks on the special counsel sound increasingly desperate.

He responded to Mr Cohen’s plea with fury. He lambasted his former lawyer and fixer as “weak” and accused him of lying. He then added that “even if he was right, it doesn’t matter because I was allowed to do whatever I wanted during the campaign.” Sooner or later, Republicans politicians may be called on to decide whether or not that is true.