Re: Locking down elections
Lobbying is first amendment protected in the most obvious and fundamental sense. Bribery is not, but an implied accusation of bribery requires at least evidence of the criminal act. It almost always is hard to be sure whether campaign contributions, by far the most common referent in this context, flow to politicians because they already are known to favor the donor's position or, alternatively, because it is thought they will cause the recipient to change position (and vote) on a particular issue. In practice, it likely involves some of each, with the mix depending, among other things, on what the politician believes constituents think about particular issues (which may be distinct from what they actually believe). Including ancient and well-known practices like log rolling and spreading around of government projects further muddies things, to the point where attributing legislative voting behavior on individual votes is generally impractical. The simple model - contributions => election => legislative votes for contributors' benefit - is hard to prove except possibly in rare cases,
The fact is that elected officials stand for election, and often for reelection quite a few times after. Utah's senior senator Orrin Hatch is a fairly extreme example: first elected in 1976, he was reelected in 1982, 1988, 1994, 2000, 2006, and 2012. Given the vanishingly small number of reports of recent vote buying*, and the demonstrably rather weak relation of campaign advertising expenditure to electoral success, other factors probably determine voting behavior in many cases. Those "other factors" include political party attachment, belief about social issues that the usual suspects in campaign contribution discussions mostly don't care much about, and a large number of others, some praiseworthy and others decidedly not.
* "Vote buying" also is generally unenforceable, although traditionally considered effective Even in the old Chicago days when precinct committeemen passed out money (classically, $2, later a meal voucher) to induce voting, they trusted that the recipient's basic honesty and knowledge of the source would bring a vote for the "correct" candidate.