Don't go straight from the current system to your new system. Take intermediate steps.
Switch from geographic representative democracy to delegated democracy. In geographic representative democracy, you vote for a representative in a district drawn on a map. In delegated democracy, you delegate your vote to someone. The more people that delegate a vote to a particular person, the more that person's vote counts.
Why might people switch from geographic representation to delegation? Because currently we have gerrymandering, where politicians draw the districts. In a delegated democracy, there are no districts. Voters choose the person to whom they delegate their votes. And they can choose people who are more similar to them. For example, a millennial might choose Alexandria Ocasio-Cortez, while a senior citizen might choose Donald Trump.
This of course works best in a parliamentary democracy. In the United States, people might amend the constitution to create a parliamentary democracy. Or the states might stop seating electors, which would have the effect of throwing the election to the House of Representatives. Then under the new system they formalize it with a constitutional amendment (or convention).
Over time, change from a vote every two years to a system where people can change their delegate at any time. You then eliminate the secrecy of the ballot because people find it more valuable to be able to see that their vote was delegated properly than that it was secret. Also, it's hard for a ballot to be secret if you can vote from your phone.
Now have unions start asking for their members to vote for their lobbyists as delegates. At first, it is just a request. "Hey, vote for our lobbyist. She'll make sure that Congress doesn't pass laws restricting the coal that we mine and burn in our power plants. That way we won't get laid off!" Over time, it becomes a requirement. To be a member of the union, you have to delegate the vote to someone on the approved list. If not, you're out of the union.
Unions like this because it increases their power. Corporations like this because their unions want the corporation to do well so that the union can harvest money from them. The politicians that win under this system like it because, they win. The ones who lose don't have power anyway.
Over time, the corporations delegate more and more power to their unions, who can use it better. After all, they are people driven and control the votes. In the end, unions become corporate shops. I.e. the union and the corporation are one. The union owns all the stock and chooses the head of the corporation. Union members are expected to delegate their vote to the union choice. If not, they're let go from the union. And you can't work for the corporation if you're not union.
You can of course leave the union and vote for your own choice. But then you're unemployed. Since you are paid in union scrip, you lose all your savings. Retired? You lose your pension. Or you can just go along.
The problem with a single event is that the world doesn't work that way. Your single event would have to happen after a series of tiny events that you can't explain as there are too many of them.
For example, every president from George Washington through Richard Nixon had the power of impoundment. We often blame the loss of the impoundment power on Watergate. But what does a burglary have to do with spending authority? Nothing. Congress resented a series of actions taken by presidents. This made them want to eliminate impoundment for decades. They finally succeeded as a result of Watergate, which reduced presidential approval to a level where they could pass it as a good government reform.
Your single event may thus be entirely unrelated to selling votes. Selling votes may be a side effect of the event rather than the intention. That's more realistic, but it makes for lousy fiction because it looks unrealistic. By breaking it into multiple steps, you can make each step seem plausible.
- Delegation to eliminate gerrymandering.
- A parliamentary system so that the head of government is chosen by the legislature, making it easier to work together. No more split government.
- Public delegation because people want to see proof that their vote counts more than they want privacy.
- The rise of union delegation so that people can leverage their vote for better or at least continuing jobs.
- A mandate so that free loaders can't vote against the union and still keep their jobs.
- Takeover of the corporations by the unions, who have more power and influence in this system.
- The unions become the corporations.
- Only union corporations are left. Want to work? Join a union and delegate your vote.
- Everyone delegates their vote to a union corporation and the union corporations control the government.
- Rather than the union controlling the corporation, the corporation has effectively taken over the union. The union now faces corporate incentives and acts like a corporation.
Each step seems plausible. People act like that. The final steps seem inevitable from what came prior.
By contrast, starting with Citizens United requires ignoring the subsequent backlash. People aren't interested in delegating more power to the corporations now. They want to claw back some of what they've already given.