Obama buoyed in Iowa; urges supporters to vote

SIOUX CITY, Iowa (Reuters) - Democratic presidential candidate Barack Obama was buoyed on Tuesday by some good news in an Iowa opinion poll but warned his backers they need to show up and vote this week if they want him to win.

"It looks like it might work," Obama told a cheering rally in this western Iowa city, two days before Iowans launch the state-by-state nominating contests that will pick Republican and Democratic nominees for the White House race.

"It looks like we just might do this thing -- to the surprise of many."

The first-term senator from Illinois, who would become the first black president if he wins the November 2008 general election, cautioned against complacency.

"The polls look good. But understand this. The polls are not enough. The only thing that counts is whether or not you show up for caucus," he told about 750 sign-waving supporters at a gathering earlier in the day in Des Moines.

"Make the phone calls and knock on the doors and grab your friends and grab your neighbors and say it is time for us to deliver on change," Obama said.

Obama, New York Sen. Hillary Clinton and former North Carolina Sen. John Edwards have been locked in a tight race in Iowa for the Democratic nomination in the past few months.

But a poll published on Tuesday in the Des Moines Register showed Obama had opened a gap and was leading with 32 percent of likely Democratic caucus-goers, compared to 25 percent for Clinton and 24 percent for Edwards.

However, a Reuters/C-SPAN/Zogby poll of Iowa also released on Tuesday showed Clinton had a four-point edge over Obama with Edwards one point behind him.

It is notoriously hard to poll in Iowa, because of its unusual system in which voters gather in groups -- or caucuses -- to select a candidate. In 2004, former Vermont governor Howard Dean was widely expected to win the caucus but ended up in third place.

Though he was upbeat about the polls in his speeches, Obama was more reticent in comments to reporters.

"It's very hard to read what's going on except for the fact that we've got these great crowds with unbelievable energy," he said on the plane between Des Moines and Sioux City.

So many people came out for the rally in Sioux City that fire marshals limited the crowd to about 650 people, forcing a few hundred more into an overflow room.

"I think it's going to come down to who gets their supporters out and I'll put my money on my organization -- it's as good and as dedicated and as intense as I've ever seen," Obama told reporters. "I'm just going to try to help them so that they can carry us over the finish line."

Obama, who asks for a show of hands of undecided voters at the beginning of each campaign event, said he was encouraged that there were still so many undecided people at his rallies because it "gives me a chance to persuade them one last time."

(Editing by Stuart Grudgings)