That’s not the highest in the world

I never claimed it was, I merely claimed that it's substantially higher than the average person pays, which is <10% (after standard deduction and whatnot). The rich pay something like 30% after handling tax brackets and whatnot, so >3x more as a percentage of income.

If you try to increase that substantially, they'll push hard to stop it in Congress. Do you really want to spend what little political capital you have trying to eek out a bit more from them? Or would you rather spend that on something else?

Citizens United

PACs were absolutely a thing before the Citizens United case, Citizens United just made them more powerful. So even if the decision weren't made, there would still be special interests.

I disagree with the Citizens United decision, but I really don't think it's relevant here. Even if corporations were completely banned from making contributions, they'd still have a ton of power. It turns out politicians generally want to please people who can bring jobs to their area, have a captive audience (shareholders and employees), and bring in tax revenue.

Harris claimed to want to tax unrealized gains, which currently isn't legal and would be a huge uphill battle to get passed. If she was truly passionate about it, she'd have an actionable plan, but instead, she just seems to say it to get people to listen. The idea has no path to passing and she knows it.