DENVER -- Sen. Barack Obama will face a difficult balancing act when he makes his acceptance speech at Denver’s Invesco Field Thursday night, political analysts said. He will have to wow a crowd of nearly 75,000 people in the stadium while making a one-on-one connection with folks watching at home.

Crowds for the speech started arriving early in the day.

“It’s a once-in-a-lifetime experience and I wouldn’t miss it for anything,” said Obama supporter Andrea Lewis.

“This is the most phenomenal thing since Nelson Mandela,” said Obama supporter Merlin Edwards.

“I am exhilarated,” said Obama supporter Tim Glaser. “This is like changing everything. It’s like history in the making.”

Despite the party atmosphere and excitement, political veterans said Obama’s speech will have to have plenty of substance for people watching on television who may not be familiar with Obama’s political positions.

Campaign insiders said the speech carries another risk. Critics took aim at his Berlin speech as over the top and have already zeroed in on Thursday’s classically-inspired stage set and gigantic venue as another symbol of excess.

Obama supporters dismiss that criticism, saying it’s a weak argument for anyone to vote for Obama’s rival, Sen. John McCain.

Polls show Obama leading McCain by an average of just less than three points, but candidates usually see a bump in the polls after their conventions. McCain backers said his plans to introduce his running mate on Friday could minimize Obama’s bump.