SPRINGFIELD, Ill., Feb. 10 — Senator Barack Obama of Illinois, standing before the Old State Capitol where Abraham Lincoln began his political career, announced his candidacy for the White House on Saturday by presenting himself as an agent of generational change who could transform a government hobbled by cynicism, petty corruption and “a smallness of our politics.”

“The time for that politics is over,” Mr. Obama said. “It is through. It’s time to turn the page.”

Wearing an overcoat but gloveless on a frigid morning, Mr. Obama invoked a speech Lincoln gave here in 1858 condemning slavery — “a house divided against itself cannot stand” — as he started his campaign to become the nation’s first black president.

Speaking smoothly and comfortably, Mr. Obama offered a generational call to arms, portraying his campaign less as a candidacy and more as a movement. “Each and every time, a new generation has risen up and done what’s needed to be done,” he said. “Today we are called once more, and it is time for our generation to answer that call.”

It was the latest step in a journey rich with historic possibilities and symbolism. Thousands of people packed the town square to witness it, shivering in the single-digit frostiness until Mr. Obama appeared, trailed by his wife, Michelle, and two young daughters. (“I wasn’t too cold,” Mr. Obama said later, grinning as he acknowledged a heating device had been positioned at his feet, out of the audience’s view.)