Maine standard deduction should not adopt OBBB increase due to fixed-date IRC conformity

[PavelMakarchuk]

Summary

Maine uses fixed-date IRC conformity (IRC as of December 31, 2024) per 36 MRSA §111. The OBBB standard deduction increase ($15,000 → $15,750 for single filers in 2025) should therefore not apply to Maine.

Currently, me_deductions.py pulls the federal standard_deduction directly (line 23), which was updated to $15,750 post-OBBB. Maine should still use $15,000.

Linked TAXSIM issue: PolicyEngine/policyengine-taxsim#790

Three-way comparison

Single, age 54, ME, $100K self-employment income, 2025:

Variable	PolicyEngine	TaxAct (Form 1040ME)
ME AGI	$92,935	$92,935
ME Std Deduction	$15,750	$15,000
ME Exemption	$5,150	$5,150
ME Taxable Income	$72,035	$72,785
ME Income Tax	$4,642	$4,695

Root cause

me_deductions.py line 23:

standard_deduction = tax_unit("standard_deduction", period)

This pulls the federal standard deduction, which was updated for OBBB. Maine's fixed-date conformity (12/31/2024) means it should use the pre-OBBB value of $15,000 for 2025.

Suggested fix

Add a Maine-specific standard deduction parameter or override in me_deductions.py that uses the pre-OBBB federal amount ($15,000 for single, $30,000 for joint) for 2025+, reflecting Maine's conformity date.

Note: This issue may affect other states with fixed-date IRC conformity that have not adopted the OBBB.

Integration test

- name: ME single 2025 100K self-employment (TaxAct verified, taxsim issue 790)
  absolute_error_margin: 1
  period: 2025
  input:
    people:
      person1:
        age: 54
        self_employment_income: 100_000
        is_tax_unit_head: true
    tax_units:
      tax_unit:
        members: [person1]
        premium_tax_credit: 0
        local_income_tax: 0
        state_sales_tax: 0
    spm_units:
      spm_unit:
        members: [person1]
        snap: 0
        tanf: 0
    households:
      household:
        members: [person1]
        state_fips: 23
  output:
    me_deductions: 15_000
    me_taxable_income: 72_785
    me_income_tax: 4_695