Presumably wealthy parents who already send their children to high per pupil funded private schools will expect a return on their investment in the pro-voucher slate; and a great deal it is too.  A family with 2 children could get say a $10,000 voucher each.  So an investment of $5000 in the pro-voucher campaigns could net them a 300% return just the first year.  For a 4 year term, that $5000 investment could net the family $80,000 in lower tuition costs, a 1600% return on investment.
And the per pupil funding their children will benefit from will dwarf the per pupil funding for public schools.