ARNOLD, Missouri (Reuters) - When he strayed slightly off script this week on the campaign trail, the usually loquacious but gaffe-prone Joe Biden quickly reeled himself in.

U.S. Democratic vice presidential nominee Senator Joe Biden (D-DE) waves to the crowd as he introduces presidential nominee Senator Barack Obama (D-IL) at a campaign rally in Sunrise, Florida, October 29, 2008. Obama is campaigning in North Carolina and Florida on Wednesday before the November 4 presidential election. REUTERS/Jason Reed

Famous for off-the-cuff comments he later regrets and for long-winded speeches, the Democratic vice presidential candidate, a veteran foreign policy expert in the U.S. Senate, is showing unusual restraint.

“That’s what you call getting off message,” Biden quipped in a light-hearted reference to his new discipline after he took his eyes off a teleprompter for a second to greet band members in a high school gym in Arnold, Missouri, on Thursday.

With polls giving Democratic presidential candidate Barack Obama a lead over Republican candidate John McCain, the Delaware senator is anxious not to rock the boat by creating an unwelcome diversion before the election on Tuesday.

Biden, 65, was downright restrained in a vice presidential debate with Republican opponent Sarah Palin this month and most of his stump speeches have been carefully scripted.

His biggest gaffe so far has been to say that if Obama wins, he will be tested in his first six months, a comment the McCain camp seized on as it argued that the first-term Illinois senator is too inexperienced to be president.

“Watch -- we’re going to have an international crisis, a generated crisis, to test the mettle of this guy,” said Biden, immediately raising red flags among Obama campaign staffers.

Obama publicly dismissed the comment as Joe’s “rhetorical flourish” and at nearly every rally since Biden has told supporters he believes the Democratic candidate has the “steel spine” to deal with any problem that comes his way.

Earlier, Biden had alarmed Democratic aides when he said that Sen. Hillary Clinton, whose supporters were still smarting from her defeat by Obama in the contest for the Democratic candidacy, might have been a better vice presidential pick.

PRESS SHY

On the trail, Biden is kept largely away from the small group of reporters traveling with him and has not held a news conference since early September.

But on Friday, in rare comments to reporters when he stopped at a fast-food restaurant in Lima, Ohio, Biden denied he had been “muzzled” by the Obama campaign.

“I have done 200 interviews. I have done half a dozen to a dozen satellite feeds every day. I am doing shows. And so no one has said anything to me about it,” Biden said.

His opponents have been waiting for Biden to trip up. McCain supporter Sen. Lindsey Graham of South Carolina has included Biden’s penchant for gaffes in his retinue of jokes.

“He’s the sound-bite machine that keeps on giving,” Graham said recently. “Keep it up, Joe!”

Biden spokesman David Wade said voters were a lot smarter than people gave them credit for and his boss was doing a great job connecting with voters. “They are very good at smelling authenticity. With Joe Biden you get the real deal,” he said.

A lawyer and seasoned debater, Biden on the campaign trail sticks largely to what is in front of him on the teleprompter.

Biden’s supporters largely forgive his missteps and refer to his foreign policy and other experience as a big plus.

“He’s human,” said Adrienne Hochberg, an Obama-Biden backer who attended a rally in Florida. “He’s a man of knowledge and experience. He can be trusted,” she said when asked what she thought of Biden’s occasional mistakes.

Biden comes from a working-class background, often bringing up his Scranton, Pennsylvania, roots. He tells supporters he knows what it’s like to struggle financially, recalling how his father lost his job and had to move to Delaware.

Aside from using his folksy style to appeal to all groups, Biden gives sharp responses to attacks on Obama. “Joe Biden has been Barack Obama’s defender-in-chief,” said Wade. “He has called his friend John McCain out for below-the-belt tactics.”

While he has made fewer gaffes than many expected, Larry Sabato, a political scientist at the University of Virginia’s Center for Politics, said they had been enough to annoy the Obama campaign and distract from the economy and other issues.

But Sabato says the biggest challenge for Obama and his No. 2 pick will likely come if they win the White House. “It is one of the easiest predictions that Biden is going to make waves from time to time that will not help Obama. That’s a given,” Sabato said.