When Donald J. Trump met Stormy Daniels, their flirtation seemed fleeting: He was a 60-year-old married mogul at the peak of reality television fame, and she was 27, not half his age, a Louisiana native raised in poverty and headed to pornographic stardom.

But that chance encounter in Lake Tahoe, Nev., some two decades ago set off a chain of events that has brought the nation the first criminal trial of an American president.

And on Tuesday, Ms. Daniels took the stand at that trial, bringing the former president face to face with the porn star at the case’s center.

The charges stem from her story of sex with Mr. Trump during that 2006 celebrity golf tournament in Lake Tahoe, a story she was shopping a decade later, in the closing days of the presidential campaign. Mr. Trump’s longtime lawyer and fixer, Michael D. Cohen, paid her $130,000 in hush money before Election Day, and the former president is accused of falsifying business records to cover up reimbursements for Mr. Cohen.

Ms. Daniels’s fast-paced testimony lasted nearly five hours, during which she described an encounter with Mr. Trump, now 77, that he has long denied. Tension gripped the courtroom, her garrulous testimony filling a heavy silence. She made jokes; they did not land.