TAMPA, Florida (Reuters) - Republicans may have made Mitt Romney's day with the presidential nomination he long sought, but it was Dirty Harry himself who nearly hijacked the show with a rambling diatribe against President Barack Obama - addressed to an empty chair.


Hollywood icon Clint Eastwood brought his star power and trademark gravelly voice to the stage of the convention hall in Tampa on Thursday, jetting in as a surprise last-minute speaker to warm up the crowd for Romney's acceptance speech.


Eastwood's cameo appearance, including an ad-libbed monologue with an imaginary Obama in an empty chair, seemed to thrill many in the audience, but was widely panned by observers across the political spectrum.


"Clint, my hero, is coming across as sad and pathetic," legendary Chicago film critic Roger Ebert said in a message on Twitter.com. "He didn't need to do this to himself."


Former Romney adviser Mike Murphy tweeted: "Note to file: Actors need a script."


The 82-year-old Academy Award-winning director and actor, who endorsed Romney earlier this month, strode to the podium serenaded by the theme music from his classic western, "The Good, The Bad and The Ugly."


Eastwood delivered an off-the-cuff, deadpan discourse, at times biting in its criticism of Obama, at times supportive of Romney's candidacy, whom he lauded for a "sterling" business record.


But more often he was nearly incoherent, meandering from one topic to another, including the state of the economy, the war in Afghanistan and the U.S. detention center at Guantanamo Bay.


At one point, Eastwood said he "never thought it was a good idea for attorneys to be president," apparently unaware that Romney holds a law degree.


In one of his lucid moments, Eastwood - squinting, with his gaunt face framed by thinning, disheveled gray hair - told the cheering crowd: "When somebody does not do the job, we've gotta let them go."


Occasionally, he paused to berate the chair, telling an absent Obama to "shut up."


The phrase "invisible Obama" went viral on the Internet, and pictures of people with empty chairs filled Twitter. Obama's own Twitter account posted a picture of Obama sitting in a chair marked "The President" with the comment, "This seat's taken."


DID CLINT BOMB?


Many felt that Eastwood bombed on the political stage.


"What the heck is THIS?" Obama campaign senior adviser David Axelrod tweeted.


"A great night for Mitt Romney just got sidetracked by Clint Eastwood. Wow. That was bad," tweeted Joe Scarborough, a former Republican congressman who currently does commentary for MSNBC.


Some in the audience, however, were left starry-eyed.


"He's a fabulous actor," said Rita Wray, a member of the Mississippi delegation, who praised Eastwood's "dry wit." She said she was a fan of his movies, though she couldn't name a single one.


It took some coaxing from the crowd, but Eastwood finally led the delegates in declaring "Make my day" - the signature line of the gun-slinging detective he played in the "Dirty Harry" movies.


Eastwood was reluctantly drawn into the 2012 campaign earlier this year when an ad by Chrysler, titled "Halftime in America" and narrated by Eastwood, ran during halftime of the Super Bowl.


Many people saw it as Eastwood promoting, and possibly endorsing, the Democratic president because Chrysler had received a government bailout.


Eastwood, who backed Republican John McCain's unsuccessful 2008 presidential bid, flatly denied that, saying at the time that he was "certainly not politically affiliated with Mr. Obama."


Eastwood, a long-time Republican, has himself dabbled in politics. He served as mayor of his small upscale hometown, Carmel, California, in the 1980s.


Convention organizers preparing for the final night of the carefully scripted event had fueled buzz about a celebrity mystery speaker by leaving a spot open on the official program.


Just hours before the session began, Romney's campaign confirmed that Eastwood was coming to town. His speech came just before Florida Senator Marco Rubio introduced Romney for the biggest test of his White House bid.


Republicans have long criticized Obama for his cozy relations with a bevy of liberal Hollywood stars like George Clooney, but convention planners apparently wanted to show that they too could bring a touch of show-business glamour to bear.


Despite Eastwood's Republican affiliation, many of his views differ with the party. Though he has described himself as a fiscal conservative, he backs gay marriage, favors gun control and abortion rights and supports environmental causes.


That may reinforce some conservatives' suspicions that Romney is himself insufficiently conservative.


(Additional reporting by Patricia Zengerle, Sam Jacobs; Editing by Jim Loney and Leslie Adler)