Obama lightens up after a good week

US Democratic presidential candidate Senator Barack Obama (D-IL) leads a discussion on the economy at Vernier Software & Technology in Beaverton, Oregon May 9, 2008.

CHICAGO (Reuters) - What a difference a good week makes.

Ever since he strengthened his lead in the Democratic race for the White House last week, Barack Obama looks like a weight has been lifted off his shoulders.

In his first days back on the campaign trail since a decisive victory in North Carolina, the Illinois senator has joined in on games with reporters he has snapped at in the past, pretended to run a hurdle event at a college track meet and been surrounded at a taco shop by diners cheering him on in Spanish.

A week ago, Obama was wondering if he would lose votes after some stumbles while campaigning and after being forced to denounce his former pastor.

But he looked like he had few worries this week as he moved closer to a victory over rival Hillary Clinton for the Democratic Party's nomination and the right to face Republican John McCain in November's U.S. presidential election.

Obama received a standing ovation from about 5,000 fans then walked a victory lap around the track during a surprise visit to the Twilight Track Meet at the University of Oregon on Friday.

He grinned widely as he walked around shaking hands, posing for pictures and throwing bright yellow T-shirts into the stands. He even pretended to take up the announcer's invitation to join the men's 400-metre hurdles race.

"Excellent form" the track announcer said over the loudspeaker after Obama -- in dress shoes and slacks -- jogged up to a hurdle then stopped and stepped over it, jogged a few steps and then stopped and waved to the cheering crowd.

GEARING UP FOR MCCAIN

Obama is not yet taking an official victory lap in the Democratic election battle and has been careful to note that the race against Clinton is not yet over.

But he is clearly looking beyond the final six Democratic primary races. On Tuesday, Obama will visit Missouri -- the first trip focused on a general election swing state.

His campaign has also just launched a registration drive to sign up more Democrats voters in all 50 states ahead of the November election.

The campaign hopes the effort also will build up a group of volunteers who will form the basis of the grass-roots effort for the general election, an Obama aide said.

"If we're going to push back on the special interests and finally solve the challenges we face, we're going to need everyone to get involved," Obama said in a statement.

"Over the next six months, Vote for Change is going to bring new participants into the process, adding scores of new voices to this critical dialogue about our future," he said, adding to the implicit message that he will be the candidate facing McCain in November.

In speeches during the past few days, Obama rarely mentioned Clinton -- usually only when asked a direct question about her.

Campaign officials said it is the right time to shift gears to focus on McCain.

"Obviously the nature of the end of this is changing," Obama's communications director and long-time adviser Robert Gibbs said in an interview. "And I think us putting the emphasis on Sen. McCain is the right one."

"Democrats didn't get in this to fight each other," Gibbs said. "They got in to win a campaign against somebody like John McCain and that's obviously ultimately what we see the mission is."

(Editing by John O'Callaghan)