National polls often don't count for a lot in the primary season run for nominations. Early results in New Hampshire and Iowa can change the national numbers overnight- making a candidate who performs well in those states seem like a potential winner, drawing skeptics into his/her camp.
For example, on the Republican side, Guliani is well ahead in national polls among declared candidates, but Romney is ahead in New Hampshire and Iowa. Could he turn wins in those states into momentum for a nomination?
On the Democratic side, Clinton has been leading both in state by state races and in national polls. Is this changing? Check out the New Hampshire numbers where Obama has closed a gap that has been as wide as 20 points to create a race now tied (31% each). In South Carolina, Obama has closed a gap as wide as 24 points to take a 4 point lead (33%-29%) over Clinton. Obama still trails Clinton by 15 points in Iowa (30%-15%)- the only state where Edwards shows any real strength.
If Obama were to win or come in a very close second to Clinton in 2 of 3 early races, he may be able to get Denocrats off their 'default' position. He may seem a more viable candidate- a candidate people can not just like, or wish they could vote for, but a candidate they can envision, and vote for, as the next President of the United States.