Clint Eastwood earned plenty of bad reviews for his latest performance: a bizarre, rambling endorsement of Mitt Romney.

"Clint, my hero, is coming across as sad and pathetic," tweeted film critic Roger Ebert as Eastwood ad-libbed Thursday night to an audience of millions - and one empty chair - on stage at the Republican National Convention in Tampa, Florida. "He didn't need to do this to himself. It's unworthy of him."

Eastwood carried on a kooky, long-winded conversation with an imaginary President Barack Obama, telling him that he failed to deliver on his promises, and it's time for Romney and his running mate, Paul Ryan, to take over.

"Mr. President, how do you handle promises that you have made when you were running for election, and how do you handle them? I mean, what do you say to people?" he said at one point to the empty chair.

Twitter was instantly ablaze with comments mocking the Oscar-winning director of Unforgiven and Million Dollar Baby.

"Clint has now eclipsed the total word count of his last three films," tweeted film critic Richard Roeper during the speech, which was intended to last five minutes but went on for nearly 12.

Howard Kurtz, host of CNN's "Reliable Sources," said "Clint's empty chair act" was the "weirdest convention moment I have ever seen." Joe Scarborough, the conservative host of MSNBC's "Morning Joe," declared that "a great night for Mitt Romney just got sidetracked by Clint Eastwood."

Minutes after Eastwood began his speech, someone created an (at)InvisibleObama account on Twitter. It has already amassed 30,000 followers and counting.

"I heard that Clint Eastwood was channeling me at the RNC," tweeted comic actor Bob Newhart, known for his one-sided conversation bits. "My lawyers and I are drafting our lawsuit."

The 82-year-old actor and director also talked about Oprah Winfrey, Obama's unfulfilled promise to close the US prison at Guantanamo, the wars in Iraq and Afghanistan and lawyers. At one point, he referenced dismissing Obama and making a change.

"When somebody doesn't do the job, you gotta let 'em go," Eastwood said. The tough-guy actor of Dirty Harry fame then drew a finger across his throat.

The Obama campaign shot back afterward by tweeting a photo of the back of the president's chair, with Obama's head peeking over it, along with the line: "This seat's taken."

Romney's wife, Ann, said she appreciates Eastwood's support, even if the actor's monologue isn't earning rave reviews. Ann Romney said she did not know what to expect when Eastwood came on as a warm up act for the evening's speakers.

"He's a unique guy and he did a unique thing last night," she told "CBS This Morning."

Eastwood, a fiscal conservative who takes left-leaning stands on social issues such as gay marriage and environmental protections, made waves with conservatives earlier this year when he starred in a Super Bowl spot for Chrysler, a company that benefited from government support. Eastwood, who endorsed Romney earlier this month at a campaign event in Sun Valley, Idaho, and once served as mayor of Carmel, California, defended his appearance in the commercial, noting it had nothing to do with his politics.

Inside the convention, the crowd cheered Eastwood's entrance and shouted his famed catchphrase, "Go ahead, make my day." But backstage, stern-faced Romney aides winced at times as Eastwood's remarks stretched on. After his speech, Romney's camp defended Eastwood.

"He's an American icon," Romney spokeswoman Gail Gitcho told CNN's Piers Morgan. "You can't look at him at through the same political lens that you would other politicians. He's Clint Eastwood."

There was seemingly more discussion Thursday night on Twitter about Eastwood's awkward performance than Romney's actual acceptance speech.

"Is this a segment for 'Mrs. Eastwood and Company'?" asked Star Trek actor Zachary Quinto on Twitter, referencing the Keeping Up with the Kardashians-like E! reality series starring Eastwood's wife, Dina.

Several celebrities and comedians lightheartedly hypothesised on the micro-blogging site how Democrats could top the over-the-top routine at their own convention in Charlotte, North Carolina, next week.

"To restore balance to the universe, Obama must have Tommy Chong onstage at the DNC talking to a steak," joked Patton Oswalt.

Original Star Trek actor George Takei said he was "drafting a DNC speech to (an) imaginary Romney in an empty factory."

"Saturday Night Live" cast member Seth Myers had an entirely different idea: "(Vice President Joe) Biden has to go shirtless for DNC to top it."

For Hollywood veteran Eastwood, his chance to rebound likely comes Sept. 21 in more familiar territory. That's when his next film, the baseball drama Trouble With the Curve, opens.