Democrats Barack Obama and Hillary Clinton shared a debate stage alone for the first time lat night.
The two struck a cordial tone and highlighted their opportunity to make history as the next US president.
"Just by looking at us, you can tell we aren't more of the same," Mrs Clinton, a New York senator who would be the first woman US president, said at the final debate before Democratic nominating contests in 22 states on Tuesday. "We will change our country."
Senators Hillary Clinton and Barack Obama during last night's debate in Los Angeles
Mr Obama, an Illinois senator who would be the first black US president, said their battle for the Democratic Party's nomination in November's presidential election was a testimony to the progress of the party and country.
"We have the opportunity to make history because I think one of us two will end up being the next president of the United States of America," Mr Obama said.
The two Democratic White House contenders dropped the angry and confrontational approach of their last debate in South Carolina, directing their attacks at Republican front-runner John McCain
while disagreeing on who could best lead US troops out of Iraq.
"I was friends with Hillary Clinton before we started this campaign; I will be friends with Hillary Clinton after this campaign is over," said Mr Obama, who held Mrs Clinton's chair as she sat down to begin the debate.
Senator Barack Obama
The pair split the first four significant nominating contests, with Mr Obama (46), winning Iowa and South Carolina, and Mrs Clinton (60) winning New Hampshire and Nevada.
Mrs Clinton, asked about the prospect of another term in the White House for a Clinton after eight years of her husband and 12 years of a member of the Bush family, took aim at both Bushes.
"It did take a Clinton to clean after the first Bush and I think it might take another one to clean up after the second Bush," she said at the Kodak Theatre in Los Angeles.
It was the first debate since last week's fierce South Carolina showdown, which featured harsh personal attacks between the two senators that sparked a week of bitter accusations by Mr Obama, Mrs Clinton and Bill Clinton.