I'm here in Arizona, McCain's home state which he'll win in a walk. He got something like 80% of the vote last time he was up for reelection to the Senate (including mine -- not proud of that). So I know how he looks in a place where he's wildly popular.
It's not that I think he's going to be easy to beat nationally, but I DO think he's got some obvious weaknesses, and the biggest one is George Bush. He's aligned himself with the very unpopular lame duck President on the war, the economy, and generally everything that's made him very unpopular. McCain represents more of the same -- and this is going to be apparent in a stark visual way if he's standing next to Obama in the debates. If this election is going to be about change, let's put an old man next to a young man and see what happens, nevermind the race issue. (If Clinton's the nom -- unlikely but still possible -- it'll be a referendum on the Bush vs. the Clinton Administration. Which also gives the Dems a huge advantage.)
His next weakness is his unpopularity with the base. Big business doesn't like him because of his contributions to campaign finance reform, and fundamentalists don't like him because he's moderate on immigration, voted against the anti-gay marriage amendment, and, horror of horrors, thinks we shouldn't torture terror suspects. None of these people are going to vote for a Democrat, but they might stay home.
Now, the right-wing noise machine is working hard to neutralize that now; most of the people who spent the last year demonizing McCain have fallen in line now that he's the nom.
And of course McCain has changed his tune on a lot of these issues and fallen in line to appeal to the crazies, but that itself is a vulnerability -- remember the phrase "flip-flopper"?
All this to say, no, it won't be easy, but I give the edge to the Democrats simply because America's sick of having a Republican President. Guess we'll see.