Democrat Hillary Clinton touted her experience and rival Barack Obama made his case for change as White House hopefuls scoured Iowa for support one week before the US state's too-close-to-call nominating contest.
Mrs Clinton and Mr Obama were among 10 Democratic and Republican candidates who spent the day in Iowa, where next Thursday voters in both parties kick off the state-by-state battle to choose candidates for the November 4, next year's election to replace Republican President George W. Bush.
Polls show a tight Iowa race on both sides.
Mrs Clinton, Mr Obama and John Edwards were in a three-way fight among Democrats and Mitt Romney and Mike Huckabee battled for the Republican lead in a state where a win provides vital momentum.
Mr Obama, a first-term US senator from Illinois who has been rapped by Clinton for having too little experience for the job, said he was the candidate who could end the partisan "food fight" and accomplish real change in Washington.
In a speech that took several swipes at Mrs Clinton, a New York senator and former first lady, Mr Obama also poked fun at her husband Bill Clinton's statement that electing Obama would be a roll of the dice.
Factbox - Presidential campaign key dates
The US presidential nominating contests for Democrats and Republicans begin next week. The following charts the course to the White House, ending with the election on November 4, 2008:
December 30: The last Sunday of worship ahead of caucuses in Iowa, a state with a large church-going population. Candidates will likely spend the day seeking support from the pulpits.
January 3: Iowa holds caucuses where voters go to meetings in their neighbourhoods and pledge their support for candidates. In the Democratic contest, a candidate must receive at least 15 percent support from voters to place.
January 4: US Department of Labour issues the December 2007 employment report. This final report on the jobs picture for 2007 could give further indications of where the US economy is headed amid growing concerns among voters it is veering toward a recession.
January 5: Republican and Democratic candidates hold separate debates in Manchester, New Hampshire, trying to make a final impression on voters just three days before they go to the polls in the state's primary vote.
January 8: New Hampshire holds its Democratic and Republican primary elections. Independent voters will be key because they can vote for either a Republican or a Democrat.
January 19: South Carolina Republicans vote in their primary contest, a state where Arizona Republican Sen. John McCain is looking to avenge his loss in 2000 to the current president, George W. Bush.
January 21: Democratic candidates debate in Myrtle Beach, South Carolina, an event sponsored by the Congressional Black Caucus and CNN, just five days before voters go to the polls.
January 26: South Carolina Democrats vote in their primary election.
January 29: Florida holds its primary contests. Republican Rudy Giuliani, a former New York mayor, has staked much of his campaign on trying to win the backing of Florida's voters.
February 5: 22 states hold primary contests which could all but determine each party's nominee for president. Voting states include: Alabama, Alaska, Arizona, Arkansas, California, Colorado, Connecticut, Delaware, Georgia, Idaho (Democrats), Illinois, Kansas (Democrats), Massachusetts, Minnesota, Missouri, New Jersey, New Mexico (Democrats), New York, North Dakota, Oklahoma, Tennessee and Utah.
February 9 until June 3: Nominating contests in remaining states.
August 25-28: Democratic National Convention in Denver, Colorado.
September 1-4: Republican National Convention in Minneapolis-St. Paul, Minnesota.
November 4: US voters elect a president, vice president, all 435 members of the House of Representatives and 35 members of the 100-seat Senate.
Calendar of presidential primaries
January
3: Iowa
5: Wyoming (Republican)
8: New Hampshire
15: Michigan
19: Nevada, South Carolina (Republican)
26: South Carolina (Democratic)
29: Florida
February
1: Maine (Republican)
5: Alabama, Alaska, Arizona, Arkansas, California, Colorado, Connecticut, Delaware, Georgia, Idaho (Democratic), Illinois, Kansas (Democratic), Massachusetts, Minnesota, Missouri, New Jersey, New Mexico (Democratic), New York, North Dakota, Oklahoma, Tennessee, Utah
9: Louisiana, Kansas (Republican)
10: Maine (Democratic)
12: District of Columbia, Maryland, Virginia
19: Hawaii (Democratic), Washington, Wisconsin
According to staff with Hawaii's Republican Party, the party is not technically conducting a presidential primary or caucus. Delegates will be selected during a week-long period tentatively ending February 7.
March
4: Ohio, Rhode Island, Texas, Vermont
8: Wyoming (Democratic)
11: Mississippi
April
22: Pennsylvania
May
6: Indiana, North Carolina
13: Nebraska, West Virginia
20: Kentucky, Oregon
27: Idaho (Republican)
June
3: Montana, New Mexico (Republican), South Dakota
August
25-28: Democratic National Convention in Denver
September
1-4: Republican National Convention in Minneapolis-St. Paul, Minnesota
Note: The Kansas State Legislature opted not to fund a presidential primary election; parties will hold caucuses instead.