Democrat Barack Obama is riding a burst of momentum into today?s US presidential nominating contests with a string of weekend wins, while Republican John McCain received praise from onetime rival President George W. Bush as he tries to woo conservatives.

Locked in a deadlocked state-by-state battle with Obama for the Democratic nomination, Senator Hillary Clinton replaced her campaign manager after Saturday?s losses to the Illinois senator.

Obama, who would be the first black president, scored a win in Maine on Sunday after sweeping caucuses in Louisiana, Nebraska and Washington a day earlier. ?We have now won on the Atlantic coast, we've won on the North Coast, we've won on the Pacific Coast, and we've won in between those coasts,? Obama said at a rally in Virginia Beach, Virginia after the Maine results were announced.

The Clinton staff shake-up came before the ?Potomac Primary? today when both parties hold contests in the US capital and in neighboring Maryland and Virginia. At a rally at a university in Bowie, Maryland, Clinton told the audience their choice ?matters this year more than ever? and they should ?pick a Democratic nominee who has been tested and vetted and can go the distance against John McCain.?

Clinton, a New York senator who would be the first woman president, did not mention Obama?s Maine victory nor did she discuss the staff shake-up.

Clinton named Maggie Williams, a top aide when she was first lady, to take over as campaign manager from Patti Solis Doyle, who was moved into the role of senior adviser.

Larry Sabato, a political science professor at the University of Virginia, said the shuffle indicated Clinton and her aides are concerned about the direction of her campaign.

?If you?re the ocean liner of the Hillary Clinton campaign and you?re trying to change course, this is how you would do it,? he said. ?All of the pieces are changing.?

Clinton and Obama have been about even in pledged delegates but well short of the 2,025 needed to win the Democratic nomination for November?s presidential election.