Every individual (other than a nonresident alien) whose income tax liability for the taxable year is $3 or more may designate that $3 shall be paid over to the Presidential Election Campaign Fund in accordance with the provisions of section 9006 (a). In the case of a joint return of husband and wife having an income tax liability of $6 or more, each spouse may designate that $3 shall be paid to the fund.

For purposes of subsection (a), the income tax liability of an individual for any taxable year is the amount of the tax imposed by chapter 1 on such individual for such taxable year (as shown on his return), reduced by the sum of the credits (as shown in his return) allowable under part IV of subchapter A of chapter 1 (other than subpart C thereof).

A designation under subsection (a) may be made with respect to any taxable year—