As Ronald Reagan's lead in the polls holds steady or increases, instead of heading toward zero as the Democrats hoped it would, you start to hear a lot of second-guessing of Walter Mondale's campaign. Some of it's obviously valid: of course he should not have appeared before sparse crowds at 9 a.m. Labor Day morning in New York; of course he shouldn't have gotten himself in a spot where he had noth do than squeeze the Charmin, as he did in Neenah, Wis. But when you go from mechanics and campaign tactics to the overall strategic decisions Mondale has made, you ought to do one thing before you start second-guessing, and that is to consider what his alternatives were.

Take three major strategic decisions made this summer: the selection of Geraldine Ferraro, the concentration early in the summer on campaigning in the South and (even still) California, and the tax increase proposal. None of them looks like a net vote-winner today. Ferraro's selection also brought the campaign three weeks of diversion while the press speculated about her personal finances -- and didn't bring over enough women to balance off Reagan's lead among men. Almost all of the South remains out of reach for the ticket, and concentrating there may have prevented Mondale from nailing down his industrial-state base, which is still threatening to go for Reagan. California, despite a relatively high Mondale-Ferraro percentage, still has a higher Reagan percentage. And you can judge who is helped by the tax increase proposal when you note that the Democrats are no longer running TV spots on it and the Republicans are.

You could make one more argument against all these decisions: that each reflects Walter Mondale's liabilities. You could say that the choice of Ferraro shows he buckles under to pressure from outre groups, the pursuit of the South demonstrates that he takes for granted a Democratic majority coalition that no longer exists, and the tax increase proves that he speaks the language of Washington policy mavens and not that of ordinary voters.

I'll grant you all these criticisms, but still argue that second-guessing is out of order. Mondale's strategic choices were sensible, prudent gambles when he made them.

For he wasn't playing with a terribly strong hand. When the Democrats met in San Francisco in July, Reagan had been winning 54 to 49 percent in polls against Mondale since the beginning of February. He was an incumbent president credited with strong leadership running in a time of peace and prosperity. Let inertia take over, and he would be reelected. So Mondale had to do something to shake up the situation if he was to win.

He was like a poker player with a pair of twos who checks, sees another player open and then drw two cards. The other guy obviously has a high pair or maybe three of a kind; the high pair has him beat, and almost nothing he draws will beat three of a kind. If he's going to stay in, he might as well draw four cards.

That's what Mondale did in July and August. This usually risk-averse politician took some breathtaking gambles -- though he was careful to take gambles that were consistent with his own policies and thinking. They had some success -- and might plausibly have had more. Ferraro has helped in some places and at some times; the South was perhaps worth a look-see; Mondale strategists continue to believe they could carry California in a close election; the tax increase was a reasonable attempt to get people's minds off the 1984 economy, which helps Reagan, and onto the 1985 deficit, which might help Mondale. But none of the gambles paid off big -- unless everyone but the ghost of Harry Truman is wrong about where this election is headed.

It's true that second-guessers can come up with some other gambles and argue that they would have worked better. Some strategies would probably have netted Mondale more electoral votes than he seems to have sewed up now -- but not nearly the 270 needed to win. But one thing I haven't yet seen is a sure-fire strategy for a challenger to beat a peace-prosperity- and-strong-leadership incumbent president. If you've got one, there are some folks over at 2201 Wisconsin Avenue who would still be interested, I think, in having a look at it.