When a politician breaks a campaign promise, aren't there measurable damages to voters and donors? When a politician says something to make money, and doesn't do it, how is that different from fraud?
Because there is no duty to be honest
Generally, there is no duty of honesty except in specific circumstances. These include:
- when under penalty of perjury
- when negotiating a contract
- when immediate and direct harm would result from dishonesty (e.g. telling someone it was safe to cross the road when it wasn’t)
- under most consumer protection laws, when engaged in trade or commerce (which politics isn’t)
Definitions differ but fraud requires both dishonesty and deception to acquire property or money or to cause loss. A vote is not property or money. While donations involve money, they are gifts and are given without expectation of reciprocity.
Typically the liability comes in re-elections. If the people think the politician lied about his agenda to get their votes, they can kick him out and vote for his opponant. How quick this happens is dependant on how likely the constituents are to flip, how badly you did your job promised, and what the population wants the person taking the job to do during the term (usually refered to as a Mandate). In the U.S. Federally, the House of Representatives are the most likely to be dynamic, and quite prone to change, as they all must be elected every two years. Next most responsive is the President, which must be elected once every four years. Least reactive is the Senate, which is elected every 6 years (though 1/3rd of the seats are up for re-election every two years, with almost every state having both senators in seperate cycles such that they will not be up for vote at the same time. 2020 Georgia has one scheduled senator election and one emergency senator election which is why they have two in that cycle).
It's also important to know which level of government they can actually make necessary legislation on. Fixing the roads is not something your National Government deals with, so you should look at state government (National government does fund Interstate Repairs, but the state is responsible for the actual repair work along their strectch of interstate. And don't go crying to your city council about immigration concerns... that's not their job.).
There is a general understanding that the best intentions of politicians (if there is such a thing) may be checked by the checks and balances of the system. One politician might say "I want A, B, and C," and another politician says "I want B, C, and D," and the best possible compromise might well be that there is money in the budget for A and D, but not B and C -- even if B and C were on both candidates' election platform.
And even if a politician has the sole power to deliver a promise, with no need to deal and compromise, circumstances change.
Many political systems have ballot propositions and referendums for situations where the voters make a direct decision. When the voters elect representatives, those representatives are expected to apply their judgement in the situation at hand.