* Ann Romney and Chris Christie top the bill
* Hurricane threatens Gulf Coast on Katrina anniversary
* Romney flies to Tampa for wife's speech (Updates with convention beginning in earnest)
By Matt Spetalnick and Sam Youngman
TAMPA, Fla., Aug 28 (Reuters) - Presidential candidate Mitt Romney flew to Tampa on Tuesday to join fellow Republicans seeking to put their shortened convention in the spotlight and prevent his message from being drowned out by a hurricane bearing down on the U.S. Gulf Coast.
Finally kicking off festivities after the storm threat delayed the convention's start by a day, delegates were poised to affirm Romney as the party's nominee in an evening capped by prime-time speeches by his wife, Ann, and New Jersey Governor Chris Christie.
Romney, who had originally planned to stay out of the spotlight until Thursday night when he accepts his party's nomination, decided to make an early appearance in the Florida host city on Tuesday to be on hand for his wife's turn at the podium.
Republicans seeking to salvage the convention faced a stiff challenge: help Romney make an aggressive, memorable argument to voters to replace President Barack Obama while being careful to avoid the appearance of unseemly celebration while the Gulf Coast was under threat from a hurricane.
Reince Priebus, chairman of the Republican National Committee, opened with a harsh indictment of Obama's economic record, saying re-electing him would mean "four more years of failure."
"We have a message for America: Elect Mitt Romney and Paul Ryan and they'll get America working again," he said to loud cheers. "We must send America's comeback team to Washington."
Delegates gathered on the Tampa waterfront for the typically festive and partisan event, though only a sparse crowd was in attendance as the convention was gaveled into session for early procedural matters. But signs of traditional fanfare were in evidence, including live music and delegates wearing funny hats and oversized buttons.
Isaac, upgraded to a Category 1 hurricane, was forecast to hit in the New Orleans area seven years after it was devastated by Hurricane Katrina - a painful reminder of how the mishandling of the disaster response under President George W. Bush stained the last Republican administration.
While Tampa was spared the brunt of Isaac's fury, a destructive landfall in Louisiana in the next day or so could create uncomfortable split-screen television images of the convention juxtaposed with the hurricane.
The Republican gathering will culminate with Romney's nationally televised acceptance speech on Thursday, the biggest speaking engagement of his political life as he heads into a 10-week sprint to the Nov. 6 election.
He spent the past few days rehearsing at his New Hampshire vacation home before heading for Tampa, where his appearance could help energize delegates as the convention gets into full swing. He is scheduled to fly to Indianapolis, Indiana, to address the American Legion on Wednesday before returning to Tampa.
Running even with Obama or slightly behind him in most polls, Romney needs a bounce in popularity from the gathering, particularly in the 10 or so politically divided "swing states," including Florida itself, likely to decide the election.
Romney's campaign sought to play down expectations of a significant "bump" in the polls that candidates traditionally enjoy after being nominated.
"I just think all bets are off about any kind of past performance being a predictor of the future," Stuart Stevens, Romney's senior adviser, told reporters, citing the uncertain impact of the storm threat and the almost back-to-back scheduling of both parties' conventions.
Obama showed he was staying on top of the hurricane situation, warning Gulf Coast residents to make preparations, but he planned to go ahead with his own campaign events in Iowa and Colorado later in the day.
The Republicans' lineup of speakers on Tuesday night are expected to rip into Obama for his economic policies, widely seen as the president's most vulnerable point, and argue that Romney, a former private equity executive, could do a better job.
Keeping the heat on Obama for his "you didn't build that" comment about small-business owners last month, convention planners have set the day's theme as "We Built It," in a bid to highlight what they see as the president's hostility toward small business.
Even so, there were signs of lingering tensions within the party that could mar the carefully choreographed gathering.
When Texas Congressman Ron Paul, one of Romney's rivals in a bitter primary race, entered the convention hall shortly before the session opened, some of his devoted followers chanted his name and "Let him speak."
The libertarian politician has not been given a speaking slot, but his son, Senator Rand Paul of Kentucky, will address the convention.
SEEKING TO HUMANIZE ROMNEY
Ann Romney's address represents a prime opportunity to humanize her husband, who is often seen as having trouble connecting with ordinary Americans. Obama's campaign has sought to exploit this by emphasizing Romney's vast wealth.
In Tampa, part of Republican officials' aim is to present Romney's biography - as a private equity executive, Massachusetts governor and leader of the 2002 Salt Lake City Winter Olympics - in a flattering way that contrasts with the waves of attacks on Romney by the president and his allies.
Ann Romney, who suffers from multiple sclerosis and survived breast cancer, is perhaps his most effective surrogate.
"Having breast cancer wasn't easy," she told CBS's "This Morning" program in an interview that could help set a more sympathetic tone for the couple. "I've had several miscarriages actually, but having multiple sclerosis was a very, very hard time in my life."
Expectations are high for the keynote speech by Christie, New Jersey's fiery governor, which is likely to be heavy on red-meat rhetoric for conservatives.
Many Romney supporters like Christie's in-your-face style, which contrasts with Romney's stiff demeanor and has made him a rising political star.
"You start turning it around tonight," Christie told ABC's "Good Morning America" program when asked how to overcome some voters' lack of enthusiasm for Romney.
Romney's chosen running mate, Wisconsin congressman Paul Ryan, tops the bill on Wednesday.
Another speaker scheduled for Tuesday, Rick Santorum, is a different matter. The former Pennsylvania senator was the last obstacle standing in Romney's path to the nomination and had delivered attack after attack on Romney on the campaign trail.
Tuesday's agenda will be dominated by the traditional roll call of state delegations, which will make Romney's nomination official, and endorsement of the party's election platform.
The platform was shaped heavily by conservatives, reflecting the party's move farther to the right, and some positions on social issues go beyond what many Republicans support.
The anti-abortion language, which makes no exceptions for rape or incest, was approved by the platform panel amid unflattering headlines caused by U.S. Representative Todd Akin, a Senate candidate in Missouri who has said women's bodies have a way to protect them from impregnation after "legitimate rape."
(Additional reporting by Steve Holland; Editing by Alistair Bell and Leslie Adler)