Today was probably the key day of the prosecution's testimony in the trial of Donald Trump. Michael Cohen, Trump's former "fixer," became the prosecution's star witness as he took the stand, since he is the one who can best tie together all the threads of the case introduced so far. Tomorrow will continue to be key, as the prosecution is likely to finish their direct questions and the defense will begin Cohen's cross-examination. The entire case could very well hinge on how the jury reacts to his testimony these two days, and whether or not they find him believable.

Cohen, of course, is a total sleazebag. Everyone admits this, even the other witnesses in the case (one of whom called him "a jerk," and all of whom testified how hard it was to interact with Cohen). He's the ultimate New York City skeezy lawyer -- almost a walking, talking Jungian archetype. You wouldn't be at all surprised to hear that Fat Tony of The Simpsons had hired Cohen to "take care of" a problem or two, to put it a little more colorfully.

The problem for Trump's defense team is that while it will indeed be pretty easy to paint Cohen as a reprehensible character, doing so might not be anywhere near enough to convince the jury of Trump's innocence. After all, Trump's pretty sleazy himself. And he isn't exactly giving his lawyers much to work with.

If Trump were some nobody who wasn't famous and hadn't been president, then any random jury might indeed buy what Trump's legal team is selling -- that Donald Trump is a paragon of virtue and has never done a single thing wrong in his life and is merely the victim of all these other people attacking him for no reason. Seriously, that's what they're going with. Trump as "family man." A Trump who deeply cared what his wife thinks of him. A man who would pay $130,000 to a woman with whom he had not actually had sex, just to protect his wife from hearing such baseless lies about him. Or, conversely, a man who inspired such loving loyalty by his aides that one would pony up this amount of money completely on his own initiative and then never tell Trump about it -- while performing actual legal services for Trump, which he was rightfully paid for.

That is the story Trump's lawyers will be telling, both in their cross-examination of Cohen and when they begin putting on their defense. And, obviously, that whole fairy tale is somewhat of a stretch (to put it mildly). Again, if Trump were some nobody that no one had ever heard of then he might be able to sell at least one juror on his fantasy story, but he's not -- everyone already knows who Trump is.

Which, after all, is more believable? That Trump got a supermarket tabloid to make "catch and kill" hush-money payouts -- and then refused to reimburse them for the cost... or that everyone around Trump loved him so much that they spontaneously -- without him ever knowing anything about it! -- ponied up six figures just to protect his innocence from liars? That Trump wanted to strike a deal with a porn star he had had sex with to buy her silence, and then tried to stiff her by delaying the payout until after the election (when, of course, the payout would never have happened), or that this was all Michael Cohen's wild scheme that he also never told Trump about? And that furthermore Trump then tried to stiff Cohen for the money and had to be browbeaten into paying it, or that Trump was simply unaware of such pedestrian details?

Now, I am not on the jury (obviously), so I have no idea what the jurors all thought of this -- or what they'll think during tomorrow's cross-examination either. But one fact stood out to me, a financial detail that could wind up being the most damning evidence in the whole case. From the New York Times liveblog of the trial, here is the relevant information (note: different reporters wrote each of these paragraphs, but I have stitched them all together):

Michael Cohen is explaining why he was owed $180,000, instead of simply $130,000 for the hush money. He says that he was owed $50,000 -- an amount he admits was exaggerated -- to pay a firm called RedFinch for "tech services." He tells this story in his book "Disloyal." At least in part, the services were Cohen getting a computer programmer to buy IP addresses in order to rig an online CNBC poll to make sure Trump ranked among the most influential business leaders alive.

[Trump Organization Chief Financial Officer] Allen Weisselberg then doubled the $180,000 to $360,000. Weisselberg, Cohen says, expected that he would lose half of that money because it would be taxed as income, and was making him whole, even after taxes.

The irony of the "grossed up" description, which Cohen says was Weisselberg's idea so Cohen could take the money as income instead of reimbursement, is it cost Trump double what it would have otherwise. Legitimate legal expenses aren't "grossed up."

This testimony right now is absolutely crucial to the prosecutors' case. They allege that the repayments to Michael Cohen were not, in fact, legal expenses as indicated on the records and instead they were the hush money payment "grossed up" (plus that they include another payment that Cohen previously made). I imagine the jury will spend a fair amount of time examining this particular portion of Cohen's testimony.

Finally, to add to this confusing sum, Cohen was offered a $60,000 bonus, bringing the total he was to be paid to $420,000. That amount was split -- as shown in Weisselberg's notes — into 12 months' worth of payments.

And to cap off this extremely important testimony, Cohen says that Weisselberg said in front of Trump that Cohen would be reimbursed completely. Cohen testifies that Trump approved the repayments and then said, "This is going to be one heck of a ride in D.C."

The whole idea of "grossing up" the money reimbursed to Cohen points directly to the fact that these were not legitimate legal expenses, and that nobody thought they were. Which is the heart of the criminal case against Trump, really.

One interesting point here is that the prosecution has indicated that after Cohen they will likely only call one more witness. Also, that this will not be Susan McDougal (the Playboy Playmate who had a long-term affair with Trump). So perhaps one final technical witness to corroborate some key fact? Or could they actually call Alan Weisselberg to the stand? Weisselberg knows where all of Trump's financial bodies are buried, after all. He could testify that Trump knew full well what he was paying Cohen for, that Trump was indeed a micromanager when it came to spending money, and that Trump routinely tried to stiff people he had promised to pay -- all of which would bolster the prosecution's case.

This of course would be an incredibly risky thing for the prosecution to do, since Weisselberg has been reluctant to testify against Trump previously and also since he is currently serving a jail term for perjury (rather inconvenient, that). This means both that the prosecution might not be confident Weisselberg would tell the truth and also that the jury might not find him very credible even if he did.

For now, though, we've still got at least one more day of Cohen's testimony to get through. Trump's lawyers will (of course) rip into Cohen like a pack of wild dogs, since the key to their strategy is to completely destroy his credibility as a witness. It's going to be vicious, that much seems certain. Trump and Cohen now hate each other with a passion, and this is the big "punch back" moment Trump's been waiting for.

If the prosecution's final witness is a quick one, then they could wrap up the presentation of their case by the end of the week (which will consist of only tomorrow and Thursday, this week). Next week, we will see Trump's legal team really spring into action, as they put on their defense. Who knows what witnesses they will call (and why)? Who knows what fantastical things they will claim? Will Trump testify himself? We will certainly see the best preview of their defense strategy tomorrow, as they attempt to legally eviscerate Cohen on the stand. Will Cohen be provoked to anger? Will he lose whatever goodwill he's built up with the jury?

As of now, the only answer to any of that is: "Stay tuned...."

-- Chris Weigant

Follow Chris on Twitter: @ChrisWeigant