St Paul, Minnesota - Joe Lieberman, a former Democratic presidential candidate and running mate of Al Gore, completed a political transformation by addressing the Republican Party's convention and making an impassioned plea for voters to support its presumptive nominee John McCain.

Lieberman said Tuesday night that it was time to put partisan politics aside, touted McCain's record as an independent and argued the Arizona senator was the presidential candidate best able to bring change to Washington.

"What, after all, is a Democrat like me doing at a Republican convention like this?" Lieberman told the delegates. "Well I'll tell you what: I'm here to support John McCain because country matters more than party."

Lieberman was speaking on the same night that unpopular President George W Bush, the man Lieberman and Gore lost to in the 2000 election, also addressed the party's nominating convention in St Paul, Minnesota, speaking by satellite.

The address by Lieberman - a strong proponent of the war in Iraq and a more hawkish foreign policy in general - highlighted his slow transition to the conservative movement since his run with Gore.

Lieberman ran as a pro-war candidate in the 2004 Democratic primary but gained little traction. Later that same year, his stance on the war also cost him the Democratic nomination for his Senate seat in his home state of Connecticut. Lieberman decided to run as an independent and was re-elected to the Senate overwhelmingly.

Despite differing on many social and domestic policy issues, Lieberman has been one of McCain's loudest advocates since the start of the 2008 general election campaign, travelling abroad with McCain and campaigning with him across the country.

Lieberman's main task Tuesday was to rebuff the Democrats' chief critique of McCain coming out of their own convention last week in Denver, Colorado: that a vote for McCain is a vote for four more years of Bush, whose approval ratings have for months hovered around 30 per cent.

"Don't be fooled by some of these political statements and advertisements," Lieberman said. "Trust me, God only made one John McCain, and he is his own man."

Lieberman also heaped criticism on the Democratic presidential candidate, Barack Obama, arguing he was an inexperienced candidate who had shown no special ability to promote bipartisanship since being elected to the Senate in 2004.

He called Obama, 47, an "eloquent young man" but said that "eloquence is no substitute for a record - not in these difficult times for America."

McCain's own campaign has long been pushing the senator's image as an anti-establishment candidate who has clashed repeatedly with Bush and the Republican Party. The endorsement of a political figure like Lieberman targets independents and moderates within the Democratic Party.

Such swing voters will be a key to winning the most hotly contested states in November. It seems no coincidence that the delegation from battleground Ohio was seated directly below the podium on the gleaming, black convention stage.

"We're front and centre," Ohio delegate Matt Riehl said. "It's Ohio, Pennsylvania and Florida, all bundled together."

Riehl called Lieberman a "great asset" who can make a "special pitch" to his former party. Riehl was impressed that Lieberman's personal respect for McCain was strong enough to make him cross political boundaries.

"It shows that Lieberman and McCain are rising above party politics," Riehl said. "He's supporting [McCain] as an American. It transcends politics. It transcends party."