Hillary Clinton has announced that she will, if elected, raise the capital-gains tax to a maximum that equals the highest income tax bracket. She hopes to promote long-term investments by penalizing short-term ones with a tax rate that gets lower the longer an investment is held, reaching the current 20% rate only after six years.

This, Ms. Clinton says, would allow a CEO to focus on the company’s true interests rather than just making the next quarter. It is, unfortunately, exactly the sort of plan you would expect from someone who has never started a company — and who doesn’t seem to know anyone who has.

The CEO of a startup is unlike the CEO of an established business. He is not the head of a chain of command: he is the spokesman or agent of a few colleagues, entrusted for the moment to represent them. The startup CEO has one primary job, which is raising money. It is the hardest thing a young company has to do — and it is an unending process.

Most germinal startups never raise any money at all. The ones that get seed funding are already breathing rarified air, and can afford perhaps a day of celebration before they start pursuing the next round.

The picture is especially tough for tech startups. A startup that builds software doesn’t have any machinery or physical supplies to auction off if the company fails. This means that banks won’t make the kind of secured business loans of the sort small companies traditionally get.

As a result, tech startups are wholly reliant on a relatively small number of investors who are looking for something more exciting than the establishment choices and are willing to take a big gamble in the hope of a big, short-term payoff. Though Ms. Clinton’s proposal would only affect those in the top income bracket, she may be surprised to learn that those are the only people who can afford to make such investments.

Professional investors think in terms of risk: they balance the likelihood of a startup’s failure against the potential payoff of its success. Increasing the tax rate reduces the effective payoff, which increases risk. Investors can lower that risk by reducing the valuation at which they are willing to invest, which means they take a larger share of the company — a straightforward transfer of risk from investors to entrepreneurs.

Ms. Clinton’s tax therefore will not be borne by wealthy investors: it comes out of the entrepreneur’s payday. The increased tax rate means a risk-equivalent decrease in the percentage of the company the entrepreneur gets to keep. And that’s just the best-case scenario.

The other option is that the tax doesn’t get paid at all, because the investor decides the increased risk isn’t worth it — the startup can’t attract funding and dies.

That sounds melodramatic, but it is no exaggeration. A startup company never has more offers than it needs; it never raises money with time spare. Even a slight change in the risk-return balance — say, the 3.8% which Obamacare quietly laid on top of the current capital-gains — kills companies, as investors and entrepreneurs see the potential upside finally shaved past the tipping point.

A tech startup has short-term potential. That is a major part of the attraction to investors, and that makes Ms. Clinton’s proposal especially damaging. In the tech world, we all hope we’ll be the next Facebook or Twitter, but you can’t pitch that to an investor. A good tech startup takes a small, simple idea and implements it beautifully.

The most direct success scenario is an acquisition by a larger company. In the app world — and this is the upside to not having physical limitations on distribution — the timescale is remarkably accelerated. A recent benchmark example was Mailbox, purchased by Dropbox just two months after it launched.

Giving investors an incentive to not to sell will hurt entrepreneurs yet again, postponing the day their sweat equity finally has tangible value, and encouraging decisions that make tax-sense rather than business-sense.

If Hillary Clinton really wants to help entrepreneurs, she should talk to some and find out what they actually want. A lower capital-gains tax — or no capital-gains tax — would be an excellent start.