> Wouldn't such a change have to be made by the same legislature with a similar majority?

Yes. The legislature voted on the additional language and it did not pass.

> The WA constitution doesn't seem to have anything to say on income tax, just a lot about property tax?

Washington state treats income as property, and property taxes must be uniform and flat within a specific property class.

Technically, this law does run afoul of that because having an exemption makes it not uniform, and per the state constitution property tax is capped at a total aggregate levy of 1% per year.

The state will use the argument that this is not a property tax, but an excise tax on the privilege of earning high income from the state's economy. They used a similar argument for the capital gains tax, and won in the WA supreme court.

But simply changing the constitution to just allow an income tax is politically impossible? The majority isn’t big enough?

Yeah, majority isn't big enough.

To modify the state constitution, you need 2/3 vote in both the house and the senate, and it also must be approved by a simple majority of voters in the next general election.

Since republicans are always unified against an income tax, dems only hold a 30-19 majority in the senate, and a 59-39 majority in the house, missing 3 and 7 votes respectively.

The plan for this tax is basically to have it challenged on purpose and leave it up to the state supreme court to re-interpret the constitution, and consider this an excise tax, which is the same way they were able to pass the state's capital gains tax.

Why not just charge it on the paying end (payroll tax)? It would be the same thing but charged from employers.

E.g my Employer pays about 125 to pay me 100 because of payroll tax. Then I receive 70 because the employer also pays my tax directly. My income was only 100 and the total tax was 55.

Washington already has payroll taxes and revenue(!) taxes, which have been increasing. It was causing employers to leave. The State had to limit ambitions on several of these taxes because it was causing businesses to move to other States.

For highly paid tech employees, the total tax burden across employer and employee in Washington has become one of the highest in the US. Even if the employee doesn’t see all of that, the employer definitely does. Washington already has de facto income taxes by proxy and this is on top of that.

What’s the point of trying to push through an income tax if it could have been an easy raise on payroll?