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.