Paul Callan is a CNN legal analyst, a former New York homicide prosecutor and of counsel to the New York law firm of Edelman & Edelman PC , focusing on wrongful conviction and civil rights cases. Follow him on Twitter @paulcallan . The opinions expressed in this commentary are his own. View more opinion on CNN.

(CNN) To understand the true meaning of what President Donald Trump told Ukrainian President Volodymyr Zelensky in their phone conversation in July, it is important to remember that the President speaks in code.

His longtime former lawyer Michael Cohen told Congress last February that Mr. Trump had never explicitly asked him to lie. "He doesn't give you questions, he doesn't give you orders," Cohen said. "He speaks in a code. And I understand the code, because I've been around him for a decade..."

The account of the half-hour conversation that the White House released on Wednesday suggests that the Ukrainian President understood the code as well. American politicians who raise enormous sums for their campaigns without explicitly promising donors an illegal "quid pro quo," are also fluent in the unspoken language of their trade.

If House Speaker Nancy Pelosi is serious about pursuing an impeachment case against Trump, she will have to employ expert witnesses who can translate Trump's code into plain English. She will have no problem finding them on Capitol Hill.

Prosecutors do this all the time to help jurors understand what's really going on in wiretapped conversations between seasoned and wary gangsters. Any fan of "The Sopranos" would know that someone who has been "clipped, whacked , erased, iced or hit," has been murdered.

If Pelosi hopes to prove Trump did anything impeachable on the call, a careful deconstruction of his words will be required, beginning with the transcript memo released by the White House. The President's personal lawyer, Rudy Giuliani, quite helpfully provided a definition of the crime of bribery in two fiery encounters on cable news with CNN's Chris Cuomo and Fox's Laura Ingraham earlier this week. "The crime of bribery is if I offer something of value to someone in return for their official action," he told CNN's Chris Cuomo.

He expanded on this for Fox's Laura Ingraham: "The crime of bribery is defined in the Ukraine, as well as all of the world, as the following: You cannot offer something of value in exchange for official action."

Unfortunately for Giuliani, this appears to be exactly what the President did.

The White House's "memorandum" begins innocently enough, with Trump congratulating the Ukrainian President on his recent electoral victory. Zelensky then fawns over Trump. He notes that he copied Trump campaign themes such as "draining the swamp" and compliments Trump as "... a great teacher for us..."

Trump then reminds Zelensky that: "Well it's very nice of you to say that. I will say that we do a lot for Ukraine. We spend a lot of effort and a lot of time. Much more than the European countries are doing and they should be helping you more than they are."

Trump then trash-talks German Chancellor Angela Merkel, saying that "...she talks Ukraine, but she doesn't do anything."

Next, he bashes other European countries in general ("they should be helping you more than they are"), reminding Zelensky once more "but the United States has been very good to Ukraine. I wouldn't say that it's reciprocal because things are happening that are not good, but the United States has been very, very good to Ukraine."

He follows with the barely coded language: "we've supported you, unlike Germany and the Europeans." And then there is the strange language about reciprocity, about the fact that "things are happening that are not good but the United States has been very, very good to Ukraine."

Zelensky now sounds worried. He may not be a student of Trump's political road show rallies and is probably having trouble following Trump's meandering stream of consciousness. Given what Trump has just said in the conversation, Zelensky could well be thinking: What is he talking about? What have we not been cooperative (reciprocal) about? Is Trump considering a permanent cut-off of American aid and support?

A nervous Zelensky replies, reaffirming Ukrainian support and cooperation with the US, including support for US sanctions against Russia and even a Ukrainian decision to buy "more Javelins from the United States for defense purposes."

Sensing weakness, Trump now follows with the "ask." He opens rather explicitly: "I would like you to do us a favor though because our country has been through a lot and Ukraine knows a lot about it."

He then makes a glancing reference to "CrowdStrike," a company involved in the investigation of the hack of the DNC servers in 2016. And he follows with a request that Zelensky and Ukraine should cooperate with Rudolph Giuliani and the US attorney general, who will be calling him about getting "to the bottom of" whether former Vice President Joe Biden, Trump's political rival, interfered in Ukrainian affairs to compel the firing of a prosecutor who was investigating a company that employed Biden's son, Hunter.

"There's a lot of talk about Biden's son," Trump says, "that Biden stopped the prosecution and a lot of people want to find out about that so whatever you can do with the Attorney General would be great." (This Biden allegation has been debunked by independent fact checkers and to date there is no evidence of criminal wrongdoing by either Joe or Hunter Biden.) Nonetheless, the perpetuation of such a charge, even if false, could seriously damage Biden in a close Democratic primary race.

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Trump says "Biden has been bragging that he stopped the prosecution so if you can look into it. ... It sounds horrible to me."

All of this certainly looks like it fits Giuliani's own definition of bribery. President Trump appears to be using his official position as President to coerce the Ukrainian President to investigate possibly false corruption claims in an attempt to smear his Democratic political rival. The "stick" or threat that can be wielded is a cut-off of US aid unless Trump gains a personal benefit, the destruction of a political rival.

This kind of case is difficult, but provable, with a careful and fair congressional investigation developing corroborative evidence.