By Chris Cillizza, CNN Editor-at-large

(CNN) -- New Jersey Sen. Bob Menendez goes on trial today in a federal court in Newark, New Jersey, on charges of bribery in regard to his relationship with a campaign donor. Menendez has, from the start, insisted that he is totally innocent. But how much legal peril is Menendez actually in? I reached out to Thomas Cooke, a former assistant state's attorney and assistant public defender in Maryland and a longtime member of the Georgetown University faculty. He is currently a distinguished teaching professor in the McDonough School of Business. Our conversation, conducted via email and only lightly edited for flow, is below.

Cillizza: The Menendez trial starts today. Explain the charges against him and how they were brought.

Cooke: The trial will involve Menendez and Dr. Salomon Melgen. The co-defendants are charged with 18 counts of fraud and bribery. The government alleges that the criminal behavior took place on a number of separate dates with a common theme -- a relationship under which the senator is alleged to have advocated, as a senator from New Jersey, issues benefiting Melgen. The charges are extremely serious and were brought after a considerable investigation and presentation to a grand jury. The grand jury issued an indictment (formal charges) on April 1. Each of the eight bribery charges alone carry up to 15 years in prison.

Both of the defendants have maintained their innocence and have entered pleas of not guilty.

In my opinion, the bribery changes are the most serious charges that can be placed against a government official.

Cillizza: What does the government need to prove Menendez did to get a conviction? What does Menendez have to prove (if anything?)

Cooke: In all criminal trials, the government has the burden of establishing guilt "beyond a reasonable doubt." The jury verdict must be unanimous. Here, the government must establish a direct connection between the financial incentives provided to Menendez and his official acts. In order to make the case, the government will establish a long list of benefits provided to Menendez by Melgen. The facts are the facts and most of them may not be in dispute. Menendez can't deny taking trips and staying at hotels, all of which were paid for by Melgen.

Once that task is completed, the government will turn its attention to the more difficult task of showing the direct connection between the benefits and conduct on the part of Menendez.

In theory, the defendant in a criminal case has nothing to prove. The burden is 100% on the government. But that being said, in some white collar criminal cases, it is essential that the defendant give his version of the facts. In some cases, the defense will attempt to prove the defendant's theory and version without calling the defendant to testify. This is often difficult and very challenging.

From what I have reviewed (and only my best guess), I would anticipate that the defendant/s will testify.

The decision to testify or not to testify will be made by the defense only after the government has completed its case and the defense determines if it is in the defendant's best interest.

Under no circumstances can a defendant be forced to testify. This is the constitutional right against self incrimination. That being said, any statements made by a defendant outside the court process may be incorporated as a part of the government's case.

The decision to testify or not is always the big mystery in a criminal case.

If the defendant elects to testify, he will be subject to cross-examination by the government.

Cillizza: Do we have any relevant historical precedent for this sort of case against a sitting US senator? If so, what do they tell us?

Cooke: It is important to note the decision of the US Supreme Court reversing the conviction of former Virginia Gov. Bob McDonnell. On June 26, 2016, the court, in sending the case back to the appeals court, noted that an official act involved a formal exercise of power and (must) be something specific and pending before a public official.

Writing for the Supreme Court, Chief Justice John G. Roberts Jr. narrowed the definition of what sort of conduct can serve as a basis of a corruption case.

I would expect that the Justice Department has studied the McDonnell case very carefully and understands the burden it has is showing the critical connection.

Cillizza: What's the timetable for a trial like this? Do we expect a decision before the end of the year? Sooner? Later?

Cooke: A trial of this nature can be divided into different stages:

(1) Pre-trial: completed

(2) Jury selection: completed

(a) 12 jurors + alternate jurors

(3) Opening statements (while optional, I expect both sides to make opening statements)

(4) The government's case (could last weeks)

The government always goes first since it has the burden of establishing guilt beyond a reasonable doubt.

There are published reports that the government intends to introduce over 20,000 pages of admissible evidence.

One word of caution (from my experience as a government prosecutor): Don't present a case that is so complicated that you lose the primary focus and attention of the jury. All it takes is for one juror to be confused and there will not be a conviction. The more simple the presentation, the easier it is for jurors to comprehend what was taking place.

(5) The defense

One never knows what witnesses, if any, the defense will call.

Even if there is a list of "potential" witnesses, the main mystery will be the decision of the individual defendants to testify or not testify.

As a former state prosecutor (Maryland), I would expect the defense to admit to a number of facts established by the government but deny there is any direct or intentional connection between the work performed by Menendez.

(6) Government's rebuttal

(7) US District Court Judge William H. Walls instructions to the jury

(8) Closing arguments by both sides

(9) Jury verdict

(1) Guilty (unanimous)

(a) The Judge will schedule a date for sentencing.

(b) The defendant's right to appeal to the US Court of Appeals, Third Circuit. The appeal can only take place after the sentencing stage.

(2) Not guilty (unanimous)

(a) Government cannot appeal. The case is over.

(3) Mistrial (jury unable to reach a unanimous verdict)

(a) The government can re-prosecute the case.

From start to finish: Anyone's guess -- likely many weeks but certainly before the end of the year. If the defendants are found guilty, it could be several months after the verdict before the actual sentencing stage (more likely late in 2017 or early 2018).

Cillizza: Finish this sentence: "Bob Menendez is in _ trouble in this trial." Now, explain.

Cooke: Bob Menendez is in very serious trouble in this trial.