What is the substantial difference between an ethical lobbyist (that is, one who just hounds a law maker all the time, but does not engage in gift-giving, favor

What is the substantial difference between an ethical lobbyist (that is, one who just hounds a law maker all the time, but does not engage in gift-giving, favor trading, political donations, etc) and a particularly engaged citizenry? The lobbyist takes a pay check? Lobbying is the engagement with members of the government to promote a certain agenda. The reason the offices of our legislators are open to the public is so that the public can go in and do exactly that. It's called "lobbying" because it is what is done in the lobbies of state houses. There is nothing wrong with lobbying as such, but the combination of campaign-donations-as-protected-speech-for-corporations with lobbying makes for a very powerful, legal flavor of corruption.