A new poll shows Senator Hillary Clinton maintaining a six-point lead over rival Barack Obama in advance of next Tuesday's presidential primary in Pennsylvania. The Democratic race is expected to continue through the end of the primary season in early June. But many Democrats and political experts are beginning to ask when the race will end. VOA National correspondent Jim Malone has more from Washington.

Senator Barack Obama continues to lead in the delegate count and in the total number of popular votes won in the battle for the Democratic Party's presidential nomination.

But Senator Hillary Clinton is counting on a victory in Pennsylvania to keep her in the race, perhaps all the way to the Democratic National Convention in late August.

"I have consistently made the case that I can win, because I believe I can win," she said. "You know, sometimes people draw the conclusion that I am saying somebody else cannot win. I can win. I know I can win. That is why I do this every day."

Clinton and her campaign supporters have pounded Obama for his description of small town Pennsylvania voters as bitter from economic struggles and clinging to guns and religion.

Obama has said he regrets his choice of words, but has stood by his main point that voters have grown weary of empty economic promises from both major political parties.

Some political analysts believe the Clinton attacks on Obama could give Republicans plenty of ammunition for the general-election campaign should Obama become the Democratic nominee.

But Obama insists he will be able to unite the party and take on the presumptive Republican nominee, Senator John McCain, in the November election.

"I am absolutely confident that come August and the convention, that the Democrats are going to be unified because they feel very strongly about the need to bring about change in the country," he said.

The Clinton campaign hopes that Obama's remarks will weaken him with the so-called superdelegates, senior Democratic office holders and party officials who attend the national convention as uncommitted delegates.

About half of the 800 superdelegates have already committed to either Obama or Clinton, and whichever candidate wins a majority of the rest will likely become the Democratic nominee.

Clinton started with a big lead among the superdelegates, but Obama has been catching up.

"And what we have seen over the last several weeks is that there has been a steady flow of super delegates going for him, and the Clinton campaign is expending all of its energy to get super delegates to hold off, rather than saying please come to us, they are just trying to get them to stop going to Obama," said Richard Wolffe, White House correspondent for Newsweek magazine and a guest on VOA's Issues in the News program.

Most experts believe it is virtually impossible for Clinton to catch Obama in the delegate count, and that the only way for her to win the nomination is by convincing enough superdelegates to support her at the convention.

But some prominent Democrats find that notion unsettling. Among them is Speaker of the House, Congresswoman Nancy Pelosi.

"It will do great harm to the Democratic Party if it is perceived that the superdelegates overturn the will of the people," she said.

Other superdelegates agree, including former President Jimmy Carter.

"If a candidate had the majority of popular votes, the majority of delegates and the majority of states, all three, then for the super delegates to vote contrary to that, I think, would be very difficult to explain," he said on ABC's This Week program.

Some Democrats fear their party could be hurt if the Obama-Clinton race continues all the way to the national nominating convention in late August, making it difficult to unite behind a nominee in time for the election in November.

National Democratic Party Chairman Howard Dean is urging the party to rally around a nominee well before the August convention.

"The only thing that could make John McCain president is a dis-unified Democratic Party, and that I will not preside over," he said. "We are going to unify this party and we are going to do that by knowing who the nominee is before we get to Denver."

Political experts say Clinton must do well in the remaining primaries in order to make the case that the nomination fight should continue. Otherwise, they say pressure will build on her to concede the nomination to Obama well before the convention.

"Only if Clinton makes dramatic gains between now and the final voting in early June, and is still behind in delegates, but has won some big victories," said Bruce Miroff, a political scientist at the State University of New York at Albany. "Otherwise, I think almost everybody in the party knows it would be an absolute disaster for the fall for the party to go into a tumultuous convention at the end of August and really have almost no time to bring the party back together again. So I would expect that unless there is a dramatic tightening of the race, that the party will congeal behind the nominee, as Howard Dean is suggesting, by the end of June."

Massachusetts Congressman Barney Frank told the Associated Press that the trailing Democratic candidate should drop out of the race by June 3, the date of the last two Democratic contests in South Dakota and Montana.

Many superdelegates may agree with Frank unless Clinton can raise fresh doubts about Obama's electability against John McCain during the next several weeks.