On Tuesday, former President Donald Trump's defense team rested its case in the New York criminal trial without calling their client to the stand to testify in his own defense.

Nor did Trump's lawyers follow through on a plan to hear testimony from an expert witness on federal campaign finance law due to the excessive limitations presiding Judge Juan Merchan imposed on that would-be witness' testimony, Fox News reported.

That expert witness, former Federal Election Commission head Bradley Smith, is now speaking out about what he would have testified to on the stand if his testimony hadn't been preemptively constrained by Merchan.

Expert witness brought in to clarify complex federal election campaign finance laws

Fox News noted that former President Trump was charged by Manhattan District Attorney Alvin Bragg with 34 counts of falsification of business records for the 2017 reimbursement of Trump's ex-attorney Michael Cohen for a $130,000 "hush money" payment to porn actress Stormy Daniels in 2016 to silence allegations of a 2006 affair with Trump via a nondisclosure agreement.

Those charges are typically misdemeanors but Bragg elevated them to felonies on a still-unexplained theory that business records were falsified to conceal some other unspecified crime, which presumably involves some alleged violation of federal election campaign finance laws.

Former FEC Commissioner Smith, who Trump likened to a "Rolls-Royce" in terms of his expertise, was brought in by the former president's defense attorneys to explain to the jury the ins and outs of campaign finance laws -- only for his testimony to be nixed at the last moment due to the substantial restrictions that Judge Merchan imposed ahead of time on what he would be allowed to testify about.

Smith was not happy that he was unable to testify about FEC laws

In a thread of X posts on Monday, Smith wrote, "Judge Merchan has so restricted my testimony that defense has decided not to call me. Now, it’s elementary that the judge instructs the jury on the law, so I understand his reluctance."

"But the Federal Election Campaign Act is very complex. Even Antonin Scalia -- a pretty smart guy, even you hate him -- once said 'this [campaign finance] law is so intricate that I can’t figure it out,'" he continued with a reference to the late Supreme Court justice.

Smith went on to outline some of what he intended to tell the jury, not about Trump's case in particular but about campaign finance laws more generally, and observed that "While judge wouldn’t let me testify on meaning of law, he allowed Michael Cohen to go on at length about whether and how his activity violated FECA. So effectively, the jury got its instructions on FECA from Michael Cohen!"

"So you’ve got a judge who contributed to Trump’s opponent presiding over a trial by a prosecutor who was elected on a vow to get Trump, for something DOJ and FEC chose not to prosecute, on a far-fetched legal theory which the prosecution has been allowed ... To repeatedly misstate the law or elicit incorrect statements of law from witnesses (and unlike Cohen’s, my testimony would not have gone to the ultimate legal issue). The judge’s bias is very evident," he wrote.

Judge didn't want an expert to clarify the complex laws

In a separate thread on Tuesday, Smith wrote, "Bragg's theory hinges on the claim that Trump tried to influence an election through 'unlawful means.' To do that, he'll have to show that Trump violated the Federal Election Campaign Act. But since neither the FEC nor DOJ sued Trump, he's got to show it on his own evidence."

"If that's the case, isn't it entirely relevant (not dispositive, but relevant) to the jury's fact-finding on that question that neither DOJ nor FEC chose to prosecute? But Judge Merchan won't allow that in," he added. "He will, though, allow in numerous references to Cohen's guilty plea, and allow Cohen to testify as to how he thinks he and Trump violated FECA -- though it appears that Cohen is a dunce about campaign finance laws. This, he admonishes the jury, is solely for 'context.' Right."

Smith also spoke with the Washington Examiner's Byron York and explained, "Judges instruct the juries on the law, and they don’t want a battle of competing experts saying here’s what the law is. They feel it’s their province to make that determination. The problem, of course, is that campaign finance law is extremely complex and just reading the statute to people isn’t really going to help them very much."

He went on at length to describe the difference between and provide examples of what constitutes "campaign expenditures" that must be reported and "personal use" expenses that can't be covered with campaign funds and therefore aren't reported, and though he wouldn't have testified to his personal opinion on the stand, he told the reporter of the allegations against Trump, "So my personal belief is that this clearly would not have been a campaign expenditure, never had to be reported, and therefore was not misreported."