Democrats urged to wrap up fight

Howard Dean wants the party to be united before November's election

US Democratic Party chairman Howard Dean has said he would like the party's presidential nominating contest decided by 1 July to avoid bitter in-fighting.

Mr Dean told US network ABC he did not want rivalry between Barack Obama and Hillary Clinton to "degenerate into a big fight at the convention" in August.

Mr Obama has begun a six-day tour of Pennsylvania, which votes on 22 April.

Meanwhile the Republican contender John McCain has aired his first advert aimed at November's general election.

The Arizona senator is effectively guaranteed to pick up his party's nomination to run for president at the Republican National Convention in September.

In the Democratic contest, Senator Obama is ahead of Senator Clinton in terms of the number of delegates won in the party's primary elections.

But the battle for the presidential nomination seems set to continue all the way to the party's national convention in late August.

'Personal criticism'

Speaking on ABC's Good Morning America show on Friday, Mr Dean urged the two Democratic rivals to focus on the general election battle rather than attacks on each other.

Mr Obama has picked up the backing of a Pennsylvania senator

"I think it would be nice to have this all done by 1 July," he said. "If we can do it sooner than that, that's all the better."

Mr Dean added that there had been "some personal criticism" in campaigning, a reference to a series of increasingly sharp exchanges between the Clinton and Obama teams.

He repeated his call for party unity on network CBS, saying: "I think the candidates have got to understand that they have an obligation to our country to unify.

"Somebody's going to lose this race with 49.8% of the vote, and that person has got to pull their supporters in behind the nominee."

As he embarked on his bus tour through Pennsylvania, the next state to vote, Mr Obama picked up the endorsement of its popular Democratic Senator Bob Casey.

Speaking at a rally in Pittsburgh, Mr Casey said Mr Obama was "uniquely qualified" to lead the nation. His backing may help Mr Obama win over the state's white working-class voters.

Mrs Clinton, who is leading in polls in Pennsylvania, has been endorsed by the state's Governor Ed Rendell, Representative John Murtha and Philadelphia Mayor Michael Nutter.

Meanwhile, Vermont Senator Patrick Leahy - who has backed Mr Obama - urged Mrs Clinton to withdraw from the race, saying she "had every right, but not a very good reason" to continue.

Pastor row

Both Democratic presidential contenders have experienced challenges on the campaign trail over the past two weeks.

Mrs Clinton was pressed over claims she made about a Bosnia trip

Mrs Clinton was forced to acknowledge that she "misspoke" when she described landing under fire on a visit to Bosnia as first lady in 1996.

Mr Obama was hit by a furore over controversial remarks made by his former pastor, the Reverend Jeremiah Wright, last week, which prompted the Illinois senator to give a major speech on race relations.

The row was revived this week when Mrs Clinton said she would not have continued to belong to Mr Wright's church given his comments.

Mr Obama suggested in an interview with ABC's The View that he would have left the Chicago church he has attended for two decades had Mr Wright not retired.

Campaign advert

A national poll by the Pew Research Center suggests Mr Obama has weathered the storm over Mr Wright's comments, with the Illinois senator winning 49% support among voters to Mrs Clinton's 39%.

Republican John McCain will hope to get a headstart on the Democrats

The poll - released on Thursday - gave both a narrow lead over Mr McCain in a general election contest.

Mr McCain may be seeking to get a headstart on his eventual Democratic rival by airing his first general election campaign advert on Friday.

The advert describes him as "the American president Americans have been waiting for" and juxtaposes footage of him campaigning with images from his time as a prisoner of war in Vietnam.

Mr McCain appeared at a fundraising event with former Republican rival Mitt Romney in Utah on Thursday.

Earlier this week, he set out plans to help the country through its current economic downturn, as did Mr Obama and Mrs Clinton.