THE TROUBLE with putting those dutiful Democrats on TV the other night to answer Ronald Reagan's speech about the tax bill was self-evident: they aren't the opposition on this one. The same can be said of an increasing number of issues on which the Democrats are either complicit, irrelevant or out to lunch. In fact, as everyone knows from the noisy fracas going on, the most vehement and articulate opponents of the tax bill are those Republicans who in the past have counted themselves as the most enthusiastic of the president's supporters, men like Rep. Kemp of New York and Rep. Rousselot of California.

Actually, support for this bill came originally from a pretty odd couple: Senate Republicans and House Democrats. All but four Senate Republicans -- and not a single Senate Democrat -- supported the Dole bill. House Democrats saw to it that a tax bill went to conference committee, and they now insist that the conference committee bill be supported by a large number of Republicans; the Democrats don't want to be in a position to be blamed for killing the bill, but they don't want to be identified with it either.

Senate Republicans and House Democrats have little in common on most issues; but what they do share is their majority status. They -- and President Reagan -- recognize that they have the responsibility to pare down a deficit that the large majority of business leaders and politicians will admit, at least privately, is perpetuating high interest rates and choking off economic recovery. Sen. Dole and other Senate Republican leaders deserve credit for first realizing the need for action and then writing a bill that could receive majority support. House Democrats and the president deserve credit for having realized, somewhat later, the need for supporting such a bill and, in the speeches by Mr. Reagan and Rep. Foley Monday night, supporting it with sound arguments.

Mr. Reagan has been accused of varying degrees of treachery, timidity and political absent-mindedness bordering on amnesia by large numbers of unhappy defectors from his ranks. Some of them portray him as an incommunicado prisoner of a couple of conniving White House aides -- hardly the description of a man in command of the office. There is a great search on for justifications, for alibis, for any kind of behavioral theory that will indicate that the president doesn't know what he is really doing and therefore couldn't possibly believe in it and so is himself without involvement or blame.

The hypotheses multiply. So do the devils -- one man/one vote, one theory/one devil. As we at The Washington Post are privileged to play a minor part in all this -- our periodic support of administration programs being taken as prima facie evidence of the essential monstrosity of those programs -- we will keep out of it. Except to say two things: surely the president's unhappy constituents should at least consider the possibility that the president, if he is half the man they thought he was, has not necessarily forgotten everything since he entered the White House -- he may in fact have learned a few things. Our second point is in the way of being an assurance to our friends over there on the right: don't worry about Ronald Reagan's having become a liberal just because a couple of Democratic Party spokesmen or unrightwing newspapers agree with something he has done. We promise: we'll tell you when that becomes a danger. Right now, fellows, it isn't even close.