Section 6C. Every individual who files a separate return may voluntarily contribute one dollar to be paid over to the State Election Campaign Fund, established by section forty-two of chapter ten. In the case of a joint return of husband and wife, each spouse may voluntarily contribute one dollar to said fund. A credit in the full amount of any contribution under this section shall be allowed against the tax imposed by this chapter; provided, that for any such return no such credit shall exceed the income tax liability for any taxable year.
A contribution made under this section may be made with respect to any taxable year at the time of filing the return of the tax imposed by this chapter for such taxable year; provided, however, that the commissioner shall prescribe the manner in which such contribution shall be made on the face of the return required by section five of chapter sixty-two C.
The provisions of this section shall apply only to residents required to file a return under this chapter.
For purposes of this section the words ''income tax liability for any taxable year'' shall mean the amount of tax imposed by this chapter reduced by sum of the credits allowed by clause (a) of section six.