WASHINGTON — Senator Johnson of South Dakota returned to work yesterday, physically weaker but saying he anticipates running for re-election next year.

Mr. Johnson, a Democrat, has been out since suffering a life-threatening brain hemorrhage nine months ago. Effects of the hemorrhage remain — he uses a scooter to get around, and his words are slow and slurred. But his mind appears sharp, and he gave every indication of wanting to stay.

“It feels good, and I’m ready to go,” he said in an interview after returning to his office. Later, he spoke on the Senate floor.

“It must already be clear to you that my speech is not 100%,” he said haltingly. “My doctors tell me that it will get there. But my thoughts are clear, and my mind is sharp, and I’m here to be a voice for South Dakota in the Senate.” The Senate majority leader, Harry Reid, gave a lengthy speech praising Mr. Johnson, his wife, and his doctors. Most senators were in the chamber to hear Mr. Johnson’s remarks, and he received two standing ovations from his colleagues. “We love you, I love you,” Mr. Reid said.

Speaking with reporters, Mr. Johnson had no problem finding answers to questions, but it occasionally took him a while to form words. He said he would measure his speech and mobility progress and make a final decision on running for re-election this fall.

“I anticipate it will be good but you never know,” he said.

His return is a relief for Democrats, who have a thin majority and need his vote. Mr. Reid wasted no time readying for the senator’s political future this week.

“In honor of his first day back in the Senate, help me welcome Tim with a contribution to his campaign,” he said in an e-mail to Democratic supporters yesterday. Mr. Johnson plans to ease his way back into office, taking each day’s busy schedule as it comes. He will undergo speech therapy three times a week.

He said his return would be “an inch at a time.”

Mr. Johnson said he is waiting to hear from Reid when he can assume the chairmanship of the Senate ethics committee, which has been asked to investigate the conduct of Senator Craig, a Republican who was arrested in a sex sting in an airport restroom.

“I’m in no hurry,” he joked. “It’s a thankless job.”

Senator Boxer, a Democrat of California, is heading that panel in Mr. Johnson’s place.

His initial return to his office was a media event, with cameras crowding around as he scootered through the door alongside Senator Thune, a Republican of South Dakota, and Rep. Stephanie Herseth Sandlin, a Democrat. Staffers cheered as he entered the office. Reporters asked how he felt.

“Okay,” he said, pausing. “Good.” Getting around the Capitol was not always easy for the senator on his first day back. He had some difficulty backing out of an elevator off the Senate floor, but finally made his way out with a little help from Senators Dorgan and Conrad, both of North Dakota.

Mr. Johnson last December underwent emergency surgery for arteriovenous malformation, a condition that causes arteries and veins in the brain to grow abnormally large, become tangled and sometimes burst.

As Mr. Johnson recovered out of the spotlight, Republicans have been largely quiet about his reelection campaign. But his return will give the GOP the opportunity to begin running against him.