By Eric Johnson and Rick Rothacker

CHARLOTTE, North Carolina (Reuters) - When it comes to politics, Mother Nature is either still undecided, or just a really angry voter.

A week after Hurricane Isaac forced Republicans to cancel the first day of their convention in Florida to nominate Mitt Romney as their presidential candidate, bad weather is crimping U.S. President Barack Obama's nomination party in North Carolina.

Heavy thunderstorms that are forecast to continue through the week forced Democrats on Wednesday to move Obama's planned acceptance speech Thursday night from a 74,000-seat outdoor football stadium to a much smaller indoor venue.

The shift to the Time Warner Cable Arena was a setback for Obama, who hoped to create a visual spectacle in Charlotte's Bank of America stadium on Thursday to rival his 2008 acceptance speech in a football stadium in Denver.

Beyond the financial hit to the party - the budget for the stadium event was roughly $5 million - it was a big letdown for the rank and file.

Tens of thousands of Obama supporters from around the country who were given tickets to the biggest speech in his campaign for the November 6 election are now being advised to watch elsewhere, an unfortunate twist for a party billing its convention as "the most open and accessible in history."

Obama volunteers Honora Price and Gayle Fleming were leaving Arlington, Virginia, to drive to Charlotte when they saw a television news ticker announcing the venue change. They had paid for their hotel and didn't think twice about heading out.

"We will be in Charlotte where the action is. So I'm fine. Will I be disappointed? Of course I'll be disappointed," said Fleming, adding "we'll make the best of it."

For several days, heavy thunderstorms knocked out electrical equipment and rain pooled on plastic tarps covering the stadium's grass field, delaying preparations for an event that was also to feature musical acts Foo Fighters and Mary J. Blige. Band Earth, Wind & Fire said it pulled out of the event because of logistical problems with the smaller venue.

"We have to put safety first," said convention spokeswomen Joanne Peters. "Anyone who's been in Charlotte the last week has seen how quickly these storms can pop up and how violent they can be."

Republicans were quick to put their own spin on the venue change, questioning whether Democrats were having trouble filling the seats in the stadium, home to the NFL's Carolina Panthers.

"Suddenly Team Obama is moving inside after questions about enthusiasm for the event. What's the real forecast for the speech? Forty percent chance of lies and scattered excuses," Republican National Committee spokeswoman Kirsten Kukowski said.

Obama campaign officials denied they had struggled to fill the seats, saying they could not risk the possibility of having to evacuate the open-air stadium in the event of thunderstorms.

"We're all disappointed because we had 65,000 ticket holders plus 19,000 people who were on the waiting list, ready and excited and fired up to hear the president deliver his speech," campaign spokeswoman Jen Psaki said.

"We know that Mother Nature doesn't always cooperate, unfortunately. We wish she did," she said.

Party organizers used social media to encourage supporters to attend one of roughly 4,000 "watch parties" across the nation. Others could turn to the Internet to watch the event.

To help ease the disappointment, Obama is set to host a conference call with supporters on Thursday, and his campaign promised all ticket holders will get a chance to see the president before the election.

Julia Hicks, a member of the Colorado delegation who is originally from North Carolina, said she had 15 family members from the area who had planned to join her at the stadium on community credentials.

"I'm from around here. Didn't anyone tell the DNC that this is hurricane season and the rainy season?" Hicks said.

Having attended Obama's speech in Denver four years ago, she said holding it in the arena is a disappointment but that delegates will adapt. "We'll just have to rev it up," she said.

One bright side for the president: he doesn't have to give an acceptance speech in a stadium named for a large bank after four years of lambasting so-called fat cats on Wall Street that need more regulation.

"The symbolism has eased a bit," said Scott Huffmon, a political science professor at Winthrop University in South Carolina. "Some folks are probably breathing a sigh of relief."

(Additional reporting by John Whitesides, Alistair Bell, Doina Chiacu, Colleen Jenkins, Jeff Mason, and Matt Spetalnick; Editing by Edward Tobin and Doina Chiacu)