WASHINGTON — As he approached his 89th birthday this year, Senator Frank R. Lautenberg of New Jersey was finally considering retirement, an option he had tried a decade earlier and hated so much that he jumped at the chance to return to political life.

 

Now he was the oldest member of the Senate. His family was urging him not to seek a sixth term. Polls were showing that voters might not give him one, and he had never liked fund-raising.

Then came Cory A. Booker, the 43-year-old mayor of Newark, whose 1.3 million followers on Twitter and robust presence on television had given him a reputation as a rising star in the Democratic Party, announcing that he would run for the Senate when Mr. Lautenberg’s term is up. He made his declaration nearly two years before Election Day, and, notably, before Mr. Lautenberg, a fellow Democrat, had announced his own intentions.

If Mr. Booker was hoping to nudge Mr. Lautenberg toward the door, his announcement has had precisely the opposite effect. Mr. Lautenberg, a fighter dubbed “swamp dog” by a past opponent, has embraced his job with new vigor. He has taken a leading role on the issues of the day: fighting for recovery money for his hurricane-hit state and pushing for gun control after the shootings in Newtown, Conn.