WALTER Mondale has his own ''secret'' plan to win the election. It's really a last-ditch plan, and it's only barely more secret than Ronald Reagan's plan to raise taxes. Mr. Reagan has always had a last-resort, tax-increase policy, from the moment he entered the White House. It remains such. What's so secret about that?
But what some top Mondale political associates are revealing today, privately , is that the Democratic candidate's main hope of turning this election campaign around is for Reagan to stumble badly.
They even concede this may well be an election Mondale cannot win of himself - and that it will take a Reagan bumble of sizable proportions to lift the Minnesotan into contention.
The Mondale people in their comeback strategy are putting their primary reliance on televised debates with Reagan to give their candidate the political lift he so desperately needs.
Akin to this is their acknowledgment that what they call the ''age issue'' may be the one factor left to win on - the hope that the expected debates might illuminate the difference in the two candidates' ages, to Mondale's advantage.
One can assume that the debates will be exceedingly important to the Mondale camp. What is significant is that Mondale now sees debating Reagan as about the only way out of his down-under position: That's something that was not previously in the public domain.
The Mondale debate strategy comes down to this:
1. By standing side by side with the President, the Democrat hopes a sizing up of the two candidates will take place by the millions of Americans watching on television.
2. Mondale is convinced he can look sharper and better informed than the President - and, hence, that he can underscore the ''age issue'' - the assertion , from the Democrats, that Reagan is getting too old for the job.
What many of the Mondale people are really hoping for, however, is that Reagan might make some major mistake in one of his answers.
They are convinced that such a mistake would do more than cause Reagan to appear weak in his knowledge of the issues: They think it would dramatically - and perhaps decisively for the President's reelection prospects - bring the age issue to the forefront.
The obvious problem with this Mondale strategy is that Reagan is still looking fit as a fiddle. And the President possesses a disarming wit and superb timing that can befuddle a younger opponent.
Further, the kind of debate that would make Reagan most vulnerable to committing a potentially fatal error simply isn't likely to be held. That would be an old-fashioned, free-for-all confrontation where the two men could stand toe to toe, questioning each other instead of simply responding to a panel of reporters.
Jim Johnson, Mondale's campaign director, who has been ''debating'' Jim Baker , Reagan's chief of staff, over the number and nature of the debates, says he's been pushing for a real, old-time debate. But he says Mr. Baker seems to be holding out for what is basically a panel show, something like an extension of ''Meet the Press.''
In a debate where the two adversaries parry and thrust verbally for an hour or so - without reporters intervening - there would indeed be a new, heavier demand placed upon the contestants, particularly for keeping their cool when the battle became heated, as almost inevitably it would.
It would be a contest when the ''better'' man, and not just the man who looked better on TV, might well be adjudged the winner.
No, Mondale's secret hope isn't too likely to pay off - or, at least, not as handsomely as he would like. He would get visibility before the vast TV audience. And his performance might be such as to help him gain some ground.
But in the relatively polite give-and-take of present-day debates, there seems little likelihood of something happening that would give Mondale the opening he is looking for.