( dpa ) - The neck-and-neck battle between Barack Obama and Hillary Clinton for the Democratic Party's presidential nomination has taken hold of the United States capital region along the Potomac river.
Tuesday's so-called Potomac primary - which includes Virginia, Maryland and Washington - comes as centre-left Democrats nationwide increasingly weigh which prospective presidential nominee has the best chance to defeat Senator John McCain, who is nearly certain to lead the Republican Party into the November general elections.
Obama told supporters Sunday at a rally in northern Virginia that his campaign for "change" and a new kind of politics attracts independent voters and even some Republicans, while Clinton's more partisan history - already as first lady during Bill Clinton's presidency - not only made her a riskier candidate but would prevent her from implementing Democratic policies as president.
"It's very hard for Senator Clinton to break out of the politics of the last 15 years," Obama said at a high school gym in Alexandria, Virginia.
"And even if you win, you don't have a working majority for change. Senator Clinton starts off with 47 per cent of the country against her. That's a hard place to start if you want to win the White House."
Clinton, who hopes to become the first woman president, argues that her experience and ability to withstand the attacks of Republicans over the years make her the best candidate against McCain.
Obama spoke and took questions during 90 minutes with an electrified crowd of more than 2,000 people Sunday afternoon in Alexandria, Virginia, within sight of Washington's landmarks just across the Potomac. He headed off for a later campaign stop in south- eastern Virginia and planned Monday rallies at the University of Maryland and in Baltimore, Maryland's largest city.
Clinton made her first campaign trip to the region Thursday and returned for her own round of rallies in southern Virginia over the weekend and on Monday in Maryland. She also drafted in her husband Bill to offer support in the contest.
Obama, vying to become the first African-American president, holds a clear edge in all three primaries according to the latest polls. His momentum was further fueled by a sweep of four states from coast to coast - Louisiana, Nebraska, Washington state and Maine - that held contests over the weekend.
In a surprise move Sunday, Clinton announced that her campaign manager Patti Solis Doyle was to be replaced by a senior advisor, Maggie Williams. Clinton's campaign insisted that the move was not a major shake-up.
Her supporters had already acknowledged last week that all the contests on February's schedule could potentially go to Obama. Clinton is hoping to make her next stand on March 4 in the Texas and Ohio primaries.
With neither side giving up much ground, the tight race nationwide has aroused strong passions on both sides and left some worried that party unity could become strained.
"This could be protracted. It could get ugly," said Bruce Danver, a 48-year-old engineer who attended Obama's rally.
Danver said that he would support either Democrat in the general election but worried that Clinton provokes a more "visceral reaction" among her detractors.
Both candidates have sought to keep their talk positive, after a racially tinged spat last month ahead of the South Carolina primary. Even with a large African-American populations voting Tuesday, race has not been a public issue this week, but it still lies below the surface.
That hasn't stopped the political jousting from spilling onto the streets of the nation's capital. One Clinton supporter, Richelle Harrison, an African-American woman in her 40s, stood with a "Hillary" sign at a busy subway station last week near the White House.
A middle-age man, also African-American, walked past carrying a computer case over his shoulder, and yelled: "Obama for Change! Hillary is just Bill's ticket back to the White House."
Harrison, a webcasting technology specialist, said it was a typical reaction.
"As an African-American, my family and friends have been saying, 'How can you do this? You should be on Obama's side'," she said. "But I truly believe she is the best candidate. I hear what (Obama) says, and I don't believe he's ready to be president."
Obama, who was only elected to the US Senate in 2004, quipped at Sunday's rally that any more time spent stewing in Washington would "boil the hope out" of him - an argument that has resonated among his own supporters.
"I think he's down to earth," said teacher Amy Roberts, 34, of Alexandria. "I actually believe him."
On the Republican side, McCain holds a commanding lead in delegates to the Republican National Convention in September but is still seeking to lock up the nomination. Recent polls give him strong leads in Virginia and Maryland over his only remaining rival, former Arkansas governor Mike Huckabee.