Given differences of phrasing, it's hard to tell how common this allegation is, but a Google search on the word "Paul" and the phrase "hand the election to Obama" returns more than 12,000 results.
The variants run from Obama winning next November if Ron Paul win the GOP's Iowa caucus in January, to Obama winning next November if Ron Paul runs as an independent or third party candidate, and everything in between. And for "if," read "because."
I'm not buying it.
First of all, a number of polls show Paul doing at least as well, if not better, than "generic Republican" or any other particular nominee versus Obama.
Secondly, the Republicans' chances really aren't that good no matter whom they nominate.
Thirdly, independent and third party candidates almost always massively over-poll their actual performance, and more so the further out from the real election that the polling is done.
Bottom line: If you vote, and if you vote Republican, and if you like Paul better than you like the other GOP candidates for whatever reason, don't let anyone sell you on the silly idea that a vote for Paul is a vote for Obama.