ONE OF THE unexpected casualties in President Reagan's tax plan is the $1 checkoff for financing presidential elections. Unexpected, because this really isn't a tax provision at all. Your tax bill is the same whether you check off the $1 or not. The revenue the government receives is the same too; the only difference is that the amount checked off is put into a fund for financing presidential campaigns. That amounts to something on the order of $35 million a year -- not enough to make any noticeable difference in the government's fiscal position.

But it's enough to make a substantial difference in our quadrennial presidential politics. The fund created is dispersed to candidates according to two formulas: in matching funds to candidates for their parties' nominations and for the major-party nominees in general elections. If the checkoff were abolished, as Mr. Reagan proposes, and the fund allowed to run dry, there would be only two ways to finance presidential campaigns, both unsatisfactory. One would be to depend on appropriations by Congress. The possibilities of political manipulations here are vast. You only have to remember the unseemly scrambling and huge pressures when a Supreme Court decision suspended operation of the campaign finance laws until Congress rewrote them during the 1976 primary season.

The other alternative is for presidential campaigns to be financed entirely by private contributions. There must be a part of Mr. Reagan that wants this: he has refused to check off $1 from his own taxes even though his campaigns have been the biggest beneficiaries in history of the public financing provisions. But nothing in the current law prevented Mr. Reagan or prevents future candidates from financing campaigns from private contributions. Most candidates (including Mr. Reagan three times) have decided to seek public funding, even though in doing so they have had to accept overall ceilings on spending and have had to meet detailed accounting requirements. They have done so for good reason: the amount of time and effort required for raising the $60 million or so needed in a presidential campaign these days would divert time and energy needed for campaigning. Is this what Mr. Reagan wants candidates to go back to?

The official answer is that the checkoff was eliminated in the Reagan plan because it takes a line on the tax return and has confused some taxpayers; you see, they say, we just wanted to simplify things. Sure. Intentionally or not, the scrapping of the checkoff makes a major change for the worse in a campaign finance law that has worked tolerably well. This isn't a tax issue at all. Whatever Congress does with the president's bill, it should keep the checkoff as it is.