If I lobby with 50% + 1 of Congressmen, is it bribery or is it lobbying?

This is correct as far as it goes. Where it starts to more closely resemble bribery is when you have the so-called "revolving door" of people who spend time working for elected officials, or perhaps holding office themselves, and who later turn around and work as lobbyists at other points in their careers. In this system, a lawmaker who does the bidding of a particular industry will make connections with the lobbying firms that industry employs and will likely be a strong candidate for a position at such firms if and when they leave public office. After all, they're excellently qualified, having served in public office, and they obviously agree with the positions they're paid to lobby for, since they advanced those agendas during their time in office.