The definition of an honest politician is one who stays bought.
Nobody in politics receives a single "campaign contribution" aka "bribe". Instead, it's an ongoing revenue stream.
Lobbyists frequently lobby on behalf of a number of companies, on a number of issues. The lobbyists know which politicians can be trusted. The politicians know which lobbyists can be trusted.
That does seem somewhat cynical, and the whole business does seem a bit seedy if not sordid. But what is the alternative? Our republic seems bad, but it functions better than most other countries out there.