Outside the courtroom, Trump is the center of attention. Inside, he has a rival.

Everywhere in our universe, a basic physical law applies: the greater the mass of an object, the stronger its gravitational field.

The accumulated mass of fame and political status places Donald J. Trump at the center of most rooms he finds himself in. In the dimly lit New York courtroom where he is standing trial, his gravitational field remains strong.

Secret Service agents are glued to his every move and gesture, as are many reporters. Jurors glance and gawk. If he were to storm out of court suddenly, as he has in other proceedings, it would be the biggest news of the day.

But in court, unlike almost everywhere else, Mr. Trump has competition: The judge, Juan M. Merchan, exudes his own gravity and has power Mr. Trump does not. And on Thursday, after a new pool of 96 prospective jurors walked into the high-ceilinged room, their attention slid from the former president seated at the defense table to the judge. Justice Merchan spoke to them for a half-hour about the case and their roles.

“The defendant in this case is Donald Trump,” the judge said, “and he is seated to my right.” Several of the Manhattanites — normally so good at pretending to be unfazed by encounters with the famous — took the opportunity to stare.

“My role,” the judge added, “is to help assure a fair and orderly trial.”

The contest for attention between the judge and the former president — the rule of law and the spectacle that butts up against it — is likely to be one of the defining characteristics of the trial, in which the former president is charged with 34 felonies, accused of covering up a sex scandal that could have hurt his presidential campaign.