This is a choice you can make at the end of your tax return. If you chose to support the presidential campaigns, it will not alter the amount of your tax return or the amount that you owe. The money collected from this choice goes to support the presidential primary and general election campaigns or it may also be used for the national party conventions.
Both Republican and Democratic parties receive fixed amount from these contributions. The amounts that each party involved in the campaigns receives are listed on en.wikipedia.org under presidential election campaign fund if you would like to know the exact amounts. Other candidates may receive a smaller amount of these contributions if they receive more than five percent of the votes. The national parties also receive a portion of these contributions to cover the costs of their national conventions. There of course are requirements in order to be eligible to receive money from theses contributions.