Republican Mitt Romney has told US voters to leave behind the "disappointment of the last four years" in a speech accepting his nomination as the party's presidential candidate.

Addressing the crowd at the Republican convention in Tampa, Florida, Romney said he would work to unify a divided country that believed Obama's campaign promises but had lost hope they would be fulfilled.

"What is needed in our country today is not complicated or profound. It doesn't take a special government commission to tell us what America needs.

"What America needs is jobs. Lots of jobs," Romney said.

The speech is the former Massachusetts governor's toughest test so far in his bid to win the White House in the November 6 election.