Californian Governor and former Hollywood star Arnold Schwarzenegger has praised the "character and leadership" of President George W Bush.
He and First Lady Laura Bush were the main attractions as Republicans rallied for a second day to urge voters to give President Bush another term in office.
Mrs Bush spoke of her husband's "strength and conviction".
Hundreds of anti-Bush demonstrators were arrested outside the convention centre in New York on Tuesday evening.
Mr Bush is still on the campaign trail ahead of his speech at the convention on Thursday.
Mr Schwarzenegger gave a barn-storming performance, focusing on his life story and his love of America, says the BBC's Justin Webb in New York.
The party will hope that uncommitted voters watching on television will have been attracted by the energy and the humour of the former film star, our correspondent notes.
As a former action film star, Mr Schwarzenegger again played on a line from one of his Terminator films as he told convention-goers: "Ladies and gentlemen, America is back.
"Back from the attack on our homeland, back from the attack on our economy, back from the attack on our way of life," he said.
"We're back because of the perseverance, character and leadership of the 43rd president of the United States - George W Bush."
Laura Bush also talked about her husband's "strength and conviction", but sought to soften the image of her husband.
She described watching Mr Bush walking on the White House lawn when he was "wrestling with these agonising decisions" about whether to go to war with Iraq.
She said Mr Bush was "a little greyer" but remained the same person she married three months after meeting him at a barbecue.
The president's 22-year-old twin daughters Barbara and Jenna also gave a humorous, five-minute address. They introduced their father, who appeared via a video-link from Pennsylvania to introduce Laura Bush to the stage.
In a reference to news stories about their underage drinking, Jenna Bush said:
"We spent the last four years trying to stay out of the spotlight.
"Sometimes we did a little better job than others."
Outside the Madison Square Garden centre, hundreds of anti-Bush protesters were arrested.
Small pockets of protesters attempting acts of civil disobedience such as stopping traffic were reported in several areas and delegates were booed as they entered the convention.
BBC News Online's correspondent in New York, Kevin Anderson, reports a heavy police presence and a tense atmosphere.
Earlier, Mr Bush tried to play down comments broadcast on Monday in which he said he did not think the war on terror could be won.
In a speech to the American Legion, the nation's largest veterans' group, Mr Bush fought back against Democrat charges he was taking a defeatist stance.
"We meet today in a time of war for our country, a war we did not start, but one that we will win," Mr Bush said.
"It's a different type of war. We may never sit down at a peace table, but make no mistake about it, we are winning and we will win," he said.
Seizing on Mr Bush's earlier comments, Democratic presidential candidate John Kerry, told a rally in Tennessee: "We can, we must and we will win the war on terror."
In another development, a controversial series of ads accusing Mr Kerry of lying about his Vietnam War record are to run nationwide.
The Swift Boat Veterans for Truth have begun rolling out the ads on cable networks including Fox and CNN, despite Mr Bush's public condemnation of all such privately-funded groups.