Now that's a qualified "bull$hit", in that for small campaign contributions, I do believe that there is a distinction between the two activities. For example, I could donate $10 to Paul Kanjorski's campaign*, and that would be a distinctly different activity than visiting him to express my displeasure with the whole ridiculous "too big to fail" nonsense he spews (which would be a form of lobbying...quite ineffective...but lobbying never the less).
Now let's take the case a large corporation that can direct large sums of money towards a particular candidate's PAC, campaign, etc. Anyone out there expect that this money is coming without strings attached?
In a nutshell here's the difference: You and I, as individuals, can financially support a candidate without the expectation that we will actually get anything from that candidate that will personally impact us. Corporations (or unions, or trade groups) contribute specifically because they expect to get something out of that "expression of free speech". As a result, I'd argue that for a large organization, lobbying and campaign contributions are intertwined in a way that makes them inseparable.
(*) For the record, I would just as soon pry my eyeballs out with rusty spoons than donate to Congressman Kanjorski's campaign. However since I can spell "Kanjorski" I decided to use his name for the example.