Democratic Sen. Hillary Clinton is losing ground to Sen. Barack Obama in a national CNN/Opinion Research Corp. poll released on the eve of critical Super Tuesday presidential primaries and caucuses. The two are virtually tied in Monday's survey, which shows the New York senator has lost a comfortable national lead she's held for months over Obama and other rivals. The survey also shows Arizona Sen. John McCain as the clear Republican front-runner. Obama, who trounced Clinton in January's South Carolina primary, garnered 49 percent of registered Democrats in Monday's poll, while Clinton trailed by just three points, a gap well...