The Democrats, starting with Mr. Harkin, practically salivated at the idea of it.

“We asked Glenn Beck to estimate the size of the crowd. He said 500,000,” Mr. Harkin said, taking a swipe at the conservative television host who held a large rally last month in Washington. “We asked Sarah Palin. She said it was so big that she could see it from Wasilla.”

It was four years ago that Mr. Harkin invited Mr. Obama, a junior senator from Illinois, to be the keynote speaker at the annual steak fry here. The response from the Democratic crowd back in 2006 was so overwhelming that one month later Mr. Obama disclosed publicly that he was considering running for president.

At the time, his candidacy was seen as a long shot, a historical point that Mr. Axelrod and Mr. Plouffe reminded the crowd of as they argued that this year’s crop of Democratic candidates can overcome their own challenging odds, too, and keep control of the House and Senate.

“It’s tougher when you got some headwinds, when the political situation is not as favorable, but that’s when we need you even more,” said Mr. Plouffe, who asked every person in the crowd to find 10 more Democrats who might not be planning to vote.

Their argument sounded reminiscent to one they often delivered in the months leading up to the Iowa caucuses in 2008, where they warned against a rush to the judgment that Mr. Obama’s campaign could never survive the juggernaut of Hillary Rodham Clinton. Mr. Obama, of course, ultimately won.

“I’m asking you to do what you’ve done before. Ignore the pollsters and the punsters and the purveyors of gloom,” Mr. Axelrod said. “Go out and tell our story and tell it proudly. Make sure people understand what the choice is.”

By the time Mr. Axelrod arrived at the fairgrounds, he had changed out of the suit and tie that he had worn during a morning interview on NBC’s “Meet The Press,” and into a red plaid shirt, complete with country western snaps, and jeans. He joined Mr. Plouffe in flipping a few steaks for a photo opportunity with Mr. Harkin and other Democrats.