With a new poll showing their race to be in a virtual dead heat nationally, the Democratic presidential candidates are making their final pitches Monday to voters in nearly two dozen states before Super Tuesday. Sen. Barack Obama has erased Sen. Hillary Clinton's lead among Democrats nationally, according to a CNN/Opinion Research Corp. poll out Monday. The two are in a virtual tie, with Obama at 49 percent and Clinton at 46 percent.With a sampling error of 4.5 percentage points, that margin is too close to say for sure which Democrat is in the lead.The survey represents a dramatic turnaround in the race from a few months ago when Clinton had a significant edge over Obama.In a January 14-17 CNN/Opinion Research poll, Clinton led Obama 42 percent to 33 percent. It also had a margin of error of plus or minus 4.5 percentage points.A national "poll of polls" calculated by CNN that averages five national polls finds Clinton ahead of Obama by two percentage points - 45 percent to 43 percent. Those five surveys were done by CNN/Opinion Research, Gallup, Pew, ABC and CBS.