By Jane Little
BBC News, Washington
The weekend's results leave the Democratic race evenly poised
It is the most exciting battle for the Democratic presidential nomination in a generation.
Don't forget, it's not that long ago that Hillary Clinton was expected to be crowned by Super Tuesday.
But Barack Obama's win in Maine on Sunday came on top of his clean sweep on Saturday, when he won in Nebraska, Washington State, Louisiana, and the US Virgin Islands.
The weekend's clean sweep added to the Illinois senator's building momentum, even as Mrs Clinton's campaign manager - Patti Solis Doyle - resigned.
Now, with the candidates virtually tied for delegates, it looks more than ever as though it will be a race to the finish to secure the Democratic Nomination.
In other words it is increasingly likely that the two candidates will go to the August Convention with neither having reached the magic number of 2,025 delegates to clinch the nomination.
With prospects of a deadlock looming large, pundits are busy predicting that the super-delegates will be the deciders.
There are 796 of these "insiders", as they're called. They include members of congress, governors, former presidents and office holders as well as unelected officials.
Their increased prominence has led to speculation of a return to the smoke-filled rooms that dominated politics before the party electoral procedures were reformed after 1968.
Both the Clinton and Obama campaigns are pushing hard to secure pledges of support from these key figures.
Mr Obama was bolstered by a clean sweep of victories this weekend
President Bill Clinton is working the phones, calling in favours from his days in the White House. And of course as a former president, he is one of the super-delegates his wife can count on.
For his part, Barack Obama has a number of governors on his side, and former presidential candidate, Senator John Kerry, is also working on recruiting his fellow voters behind the scenes.
So far the Associated Press has 213 super-delegates committed to Mrs Clinton and 139 to Mr Obama.
But that leaves 74 uncommitted and 370 unknown. And one of the prerogatives of a super-delegate is that he or she may change his or her mind at the convention, whereas normal delegates are morally obliged to reflect the popular will, at least through the first round of voting.
Biggest role since 1984
The last time the Democratic super-delegates played a decisive role was in 1984 when they backed Walter Mondale over Gary Hart. This year the competition is closer.
The headlines in Sunday's papers focused on the power of these insiders and reflected a debate over whether they should and would reflect their own views or conform to the popular verdict.
FUTURE PRIMARY CONTESTS
Virginia: 12 February
Maryland: 12 February
Wisconsin: 19 February
Ohio: 4 March
Texas: 4 March
Pennsylvania: 22 April
Barack Obama said on Friday that they should reflect the latter.
"My strong belief is that if we end up with the most states and the most pledged delegates from the most voters in the country, that it would be problematic for the political insiders to overturn the judgement of the voters," he said.
Mrs Clinton took a different line.
"Super-delegates are by design supposed to exercise independent judgement," she said.
"But of course, if Senator Obama and his campaign continue to push this position, which is really contrary to what the definition of a super-delegate has historically been, I will look forward to receiving the support of Senator Kennedy and Senator Kerry."
Both senators from Massachusetts backed Mr Obama but Mrs Clinton won the state.
Eyes on the Potomac Primaries
The candidates were campaigning hard in Virginia on Sunday ahead of the so-called Potomac Primaries.
The title refers to the river that runs through Virginia, Maryland and the District of Columbia, all of which are holding their primaries this Tuesday.
Mrs Clinton replaced her campaign manager on Sunday
Both candidates told voters they had the best chance of beating Senator John McCain.
Despite losing Kansas and Louisiana on Saturday to Mike Huckabee, the Arizona senator has become the Republican Party's presumptive nominee.
Having received a presidential vote of confidence on Sunday Mr McCain is now trying to win over social conservatives still suspicious of him.
While Barack Obama is expected to do well on Tuesday, the Clinton campaign said it was already looking ahead to key contests in March, when the delegate-rich states of Ohio and Texas vote.
It promises to be a long fight to decide which history maker - the woman or the African American - will become the Democratic Party's choice for president.