ATLANTA  Saxby Chambliss, a first-term Republican senator, was re-elected by Georgia voters on Tuesday in a substantial victory, ending Democratic hopes for a 60-vote majority in the Senate that would make it difficult for Republicans to filibuster the Obama administration’s legislative agenda.

With 96 percent of the state’s precincts reporting in the runoff election, Mr. Chambliss had 57.5 percent of the vote, and his Democratic challenger, Jim Martin, 42.5 percent. The margin was far greater than the three percentage points that separated the two men in the Nov. 4 election, when neither won the required 50 percent. Many of the Democrats who turned out last month in enthusiastic support of Barack Obama apparently did not show up at the polls on Tuesday.

“For a lot of African-American voters, the real election was last month,” said Merle Black, an expert in Southern politics at Emory University. “The importance of electing the first African-American president in history generated enormous enthusiasm. Everything else was anticlimactic.”

A little more than two million people voted in the runoff, compared with 3.7 million on Nov. 4. In heavily black Clayton County, just south of Atlanta, Mr. Martin’s vote was less than half what it was in the earlier election. Only 9.2 percent of registered Georgians cast early votes in the runoff, compared with 36 percent in the general election.