According to the Maine Instructions:
A Resident of Maine is an individual that was domiciled in Maine for the entire taxable year or maintained a permanent place of abode in Maine for the tax year and spent more than 183 days there.
A Part-Year Resident is an individual that was domiciled in Maine for part of the year but is not considered a full-year resident.
A Nonresident of Maine is an individual that was neither a resident nor part-year resident but does have Maine source income.
A "Safe Harbor" Resident of Maine is an individual that was domiciled in Maine but did not maintain a permanent place of abode in Maine. Did not spend more than 30 days in the taxable year in Maine. For income tax purposes you are considered a Nonresident.
If one spouse is a resident and the other is not and a joint federal return was filed, you may change your filing status to Single and file form NRH. Schedule NRH is located in the State Section > Basic Information portion of the program. This is only available if you filed married filing joint on the federal return.
For more information, see: