I should preface this by saying that polls in general, and especially national polls, are notorious for being hopelessly inaccurate. You have to look at who is running the poll, who they looked at, how they selected participants, how they interpreted data, how they summarize their findings, and how they present their findings. At any of these stages, and often at many of them, a polling group can manipulate its system to provide a particular outcome. And even if they don't, the knowledge that it happens is enough to call even the best possible polling institutions into question, and gives those who disagree with the results a way of arguing against them (which leads to ironic twists when those same people then praise polls that they support).
That being said, new national polling indicates that Romney is starting to slide backwards, while Obama's national lead is widening. Already, Obama can secure the election if he picks up every state that supports him, however feebily. That is a tough spot for Romney to be in, especially when we haven't even had debates yet. It's possible, as always, that things could change. Bad employment numbers, shocks to the economy, more bloodshed overseas, or any other national tragedy or setback will likely harm the President's numbers. But his lead at this point is huge.
What is causing this? Well, if you haven't been paying attention, this hasn't exactly been Romney's best week. His own base doesn't like him, and he's not winning over any moderates. He keeps saying things that are dumb or insensitive or both, he's not acting presidential, and his own VP pick is showing him up. What Romney needs is consistency and positivity, neither of which seem to be his strong suit.
Now, I'm not saying it's going to be a runaway for Obama. I highly doubt it will be. But I think that Romney is going to be a bit wanting in the debates, and I think he's starting in a low position against this President. Obama has a lot of statistics going against him: unemployment, slow job growth, unrest in the Middle East, deficit and debt talks, etc. But even with all that, he's got a huge lead on Romney in most polls. So, what gives? Either people don't care about that stuff, see that it's not really Obama's fault, or they really just don't like Romney.