WASHINGTON  It was a bookend moment.

President Obama on Monday evening became the 10th American president to call on Helen Thomas at a White House news conference. And he was the first to call on Sam Stein, a reporter for The Huffington Post, whose Internet publication sprung to life during Mr. Obama’s candidacy.

For years, Ms. Thomas has held the honorific title as dean of the White House press corps. At 88, she has been questioning presidents since John F. Kennedy.

“All right, Helen, this is my inaugural moment here,” Mr. Obama said with a grin as he turned toward Ms. Thomas’s seat in the front row of the East Room. “I’m really excited.”

Ms. Thomas, now a columnist for Hearst Newspapers, did not return the pleasantry. She plunged directly into her question, wondering if Pakistan was maintaining safe havens in Afghanistan for “these so-called terrorists.” She added, did any country in the Middle East have nuclear weapons?