Washington (CNN) Michael Cohen -- a "very weak person" in the words of President Donald Trump -- made a plea deal with special counsel Robert Mueller on Thursday that should not be ignored. It's another significant moment in a probe that appears to be picking up speed and tightening the vise around some senior members of the Trump administration.

Consider what Cohen pleaded guilty to doing: Lying to Congress about the nature and extent of his dealing with Russians over a proposed Trump Tower development in Moscow. Cohen told Congress all conversations with the Russians ceased in January 2016, right around the time when Trump first faced voters in the Iowa caucuses. Turns out that Cohen was not only still talking to the Russians through June 2016, but he was also briefing members of the Trump family on the negotiations and setting up a potential trip by the presidential nominee to Russia. (The deal fell through, and Trump never traveled to Russia.)

Cohen lied, he said in the plea agreement, to protect Trump's political interests. "I made these statements to be consistent with Individual-1's political messaging and to be loyal to Individual-1," Cohen said. ("Individual 1," in case your brain fell on the pavement before reading this, is Donald Trump.)

Why does that matter? A few reasons:

1) Trump has submitted his written responses to Mueller; he did it earlier this week. We know that Mueller asked about the Trump Tower Moscow project. Since Trump denies Cohen's version of events, can we assume he did so in the written responses to Mueller as well? And if so, can one of the two men's stories be corroborated?