TAMPA (Reuters) - Republicans will delay the start of business at their national convention in Tampa, Florida by one day due to Tropical Storm Isaac, the Republican National Committee chairman said on Saturday.

Chairman Reince Priebus said the convention - originally scheduled to be a four-day extravaganza of pomp and party celebration - would convene on Monday as scheduled but then immediately recess until Tuesday afternoon.

The convention is due to formally nominate Mitt Romney and Paul Ryan as presidential and vice presidential candidates to challenge Democratic President Barack Obama and Vice President Joe Biden in the November 6 election.

"Due to the severe weather reports for the Tampa Bay area, the Republican National Convention is going to convene on Monday August 27th and then immediately recess until Tuesday afternoon, August 28th," Priebus told reporters in a telephone briefing.

He said members of the RNC, the Romney staff and officials who are running the convention made the decision unanimously in light of the weather situation.

Tropical Storm Isaac storm has pummeled Haiti, killing at least four people and has now moved on to Cuba.

Fueled by warm Gulf waters, Isaac was forecast to strengthen into a Category 2 hurricane with 100-mph (160-kph) winds and hit the U.S. coast somewhere between the Florida Panhandle and New Orleans around midweek.

Officials expect to have a better idea of how badly the storm will impact the Tampa area by Sunday.

"The Republican convention is going to take place. We know that we will officially nominate Mitt Romney and Paul Ryan," said Priebus. He added that party officials were working with Florida state officials and emergency management to ensure the safety of everyone attending the convention.

Russ Schriefer, a representative from the Romney campaign, said they would adjust times and try to fit in as many speakers as possible in three days instead of the planned four-day event.

"I think the important thing is that, even as .. the days will be abbreviated, ... we'll absolutely be able to get our message out," Schriefer said.

"We have the opportunity to tell the American people the story of the last four years, how President Obama's failed leadership has failed this country and how Mitt Romney ... can provide a better future for Americans."

The Republican convention will bring 50,000 visitors to the Tampa-St. Petersburg area, home to well over 4 million people. Over the last few days local authorities have said they could handle the crowds and the approaching storm.

Many attendees booked earlier flights to be in place before any bad weather. Hotels said they were ready to shift party schedules or move outdoor events indoors.

The last Republican convention, in St. Paul, Minnesota, in 2008 was also delayed by a day due to a storm. That year, Hurricane Gustav hit the Louisiana coast as the convention was set to get under way.

The party, still reeling from criticism of Republican President George W. Bush's handling of devastating Hurricane Katrina in 2005, rushed to delay the meeting out of respect for Gustav's victims.

Convention president Bill Harris said the party was ready operationally for the event to begin but they were acting out of an "abundance of caution" because of the uncertainty of the weather.

"I want to make sure everyone who attends the convention is safe and everyone who lives in Florida is not unnecessarily injured by any activities taking place when a storm threatens," Harris said.

Vice President Biden, who had planned to visit Florida during the convention but who had already canceled his Tampa event on Tuesday because of the storm has decided to also cancel his other events in Orlando and St. Augustine.