SHOW ME a good loser, and I'll show you a loser"--as the saying goes. Ronald Reagan and Co. have not been good losers of last Sunday night's debate, and we suppose this could be attributed to their being neither willing nor accustomed losers. But, even as it has struck us that many of the Democrats have been far from attractive in their victory, so we think the Republican aftermath of last Sunday night's encounter has been an unedifying spectacle.

Mr. Reagan has reacted in two ways. One is that he has become increasingly shrill and testy -- a little on the nasty side. This is evident in his unusually bitter, name-calling attacks on his opponent, Walter Mondale. His other response has been to allege that Mr. Mondale had some unfair cosmetic advantage -- would you believe this? -- by virtue of having chosen TV makeup whereas the president did not. The first of these tactics is probably being secretly welcomed by the Mondale camp: if Mr. Reagan loses his affability and charm and turns nasty, he will be throwing away one of his greatest political assets. The second tactic is sheer madness. It's not just that, makeup or no, nobody will buy the proposition that Ronald Reagan, the great photo-opportunity candidate, is coming on unprogrammed and unprimped and unconcerned with the appearance of things, it's the implication of this complaint. That implication is that the president in fact thinks the look of it -- the amount of rose in the cheek -- is what voters are swayed by. For him to have offered this analysis of his defeat was to betray ideas that can do him no good.

It was also to lay off the blame, yet again, on something other than himself. The president now says the problem was that he was the incumbent -- poor fellow. All week long Republicans have been seekg staff and other scapegoats for the president's Sunday night setback. This president has had a remarkable way of skittering through office without ever being held accountable for much that went wrong. (Even this week, interestingly, some of his aides were blaming Mr. Mondale for attacking him in ways that encouraged Mr. Reagan to take wrongheaded and extreme stands to prove the Democrat wrong. It was, they were seriously charging, Mr. Mondale's fault that Mr. Reagan was saying these hopeless things about what he would or wouldn't do in a second term and casting himself in concrete on the wrong sides of various issues.)

Sore losers can be losers too: the president and his defenders make a terrible political mistake in their frantic failure of grace this week.