The liberals fear the loss of high earners who will leave the state if they cannot deduct state and local taxes on their federal returns. This is the real reason why NY, CA, and other big liberal states want to keep the deduction.
In places like suburban NY and NJ pols tell the taxpayers that their prop tax will be returned in part bt the federsl deduction. So for years school district voters have agreed to budgets and tax increases figuring they would get 25% to 33% back in deductions. Now the unions are terrigied that without the deduction the school districtz could face a tax revolt. Which would actually be meaningless since no mechanism really exists in these places to reduce spending