RealClearPolitics has a very comprehensive polling list, showing national and state data, that is worth looking over. Here is the Democratic Poll data. Here is the Republican Poll data.
For the Democrats, Hillary Clinton seems to have the edge nationally, with Obama starting to lag behind. I'm pretty sure this is due to his performances at the Debates, with Obama struggling to find his ground. Clinton seemed extremely confident, though looked coached. Edwards is slipping considerably, though holding Iowa. Edwards does not have a chance realistically with the Democratic competition being a toss up between Clinton and Obama.
We will find out how the next debates, on July 23rd, will affect the polls. In many respects, these debates become more of a rehash of the previous ones, with little or no speaking time for the realistic candidates. They should scrap those that have no chance, and then the debates will have more content on what the candidates stand for.
For the Republicans, it is Giuliani staying in the lead (with slight disturbances by Fred Thompson, who is starting to make his mark). Thompson is known nationally, yet the state data shows him nonexistent on the radar. He has been dodging his seriousness with the prospect of leading this nation, and until he shows he is committed, he will stay behind in the polls. Giuliani, still reaping the benefits of leading New York after the 9/11 attacks, instills confidence in the Republican voter (Yet a strong security over social values? Will conservative social values be the stepping stone as it was with the 2000 elections or will it be overtaken by security issues?). McCain, with his stubborn approach to everything, is dropping, and I am having doubts he will recover. Immigration reform (nobleness does not equate with being liked) has killed him in the polls. Most likely it's going to be a battle between Giuliani and Thompson (once he gets into motion), with Romney and McCain struggling to keep up.
The next debates for the Republicans is in August. That is plenty of breathing room for them to get their acts together and PR opportunities they should defiantly not miss.
We will see if both Democrats and Republicans can get over their hollow talking points and get to the core of the average Americans concern, whatever it may be.