Wherever he goes these days, Walter F. Mondale is getting advice on how to beat President Reagan, most of it in the vein of "getting tough" on the front-running incumbent.

He heard it from supporters in Portland, Ore., and other stops on the campaign trail last week, and he heard it loud and clear from Democrats back here when he stopped off over the weekend.

Much of the criticism is based on a misperception of the man and his methods.

At the most obvious level, House Speaker Thomas P. O'Neill simply didn't know what he was talking about last Wednesday when he said Mondale "has to roll up his sleeves, loosen his tie and hit him (Reagan) with the facts."

O'Neill offered that gratuitous advice here in Washington, apparently without knowing that for three straight dayMondale had been campaigning -- in both rain and sunshine -- with his sleeves rolled up and his tie tugged down.

He had hit Reagan with increasing specificity on taxes, education and arms control. But, of course, the speaker's criticism of what he assumed to be a more formal Mondale campaign style stole some of the headlines and television play awey from what Mondale had been doing.

But there is a more basic misunderstanding here, at least as I judge the Mondale I have known over the past 20 years. If there is any Democrat who does not have to be dragooned into the role of a tough partisan, it is Mondale. He is far more of a party man than most of today's politicians -- as much one as Tip O'Neill or anyone else from that fading generation of New Dealers. Ask any Minnesota Republican, and you will be told that Mondale is much more partisan in his politics than his mentor, Hubert Humphrey, more condemnatory of Republicans as a species, and less open to personal friendships across party lines.

Ask almost any of Mondale's former Senate colleagues who are Republicans and they will tell you that they found him an unrelenting adversary, unwilling to concede virtues to the opposition viewpoint even in private sessions.

As for his willingness to mix it up in personal terms with an opponent, ask Sen. Bob Dole. Dole has not forgotten -- even if some of Mondale's Democratic critics seem to have obliterated from their memory -- their vice presidential debate in 1976. Mondale measured his man, stalked him like a hunter and, when Dole made the mistake of talking about "Democrat wars" of the past, Mondale cut him to ribbons.

This inclination to go for the jugular is characteristic -- not an anomaly. More than a dozen years ago, I watched Mondale in a minor-league radio debate against a hopelessly outclassed Republican Senate challenger in Minnesota. This fellow was playing the big- spender theme, and Mondale let him develop it at length.

Then he pounced, citing example after example of "spending" bills he had supported which brought hospital rooms to Alexandria, Minn., port facilities to Duluth and on and on, until the poor Republican was waving his hands in surrender. Mondale just cut him off at the ankles. And that will be his inclination when he finally gets into debate with Reagan.

Mondale has been aching for that debate for months, if not years. His eagerness to start that debate, even before the New Hampshire primary, when the voters were more interested in weighing him against Gary Hart and John Glenn, was one of the factors that almost cost him the nomination.

Those who are pushing Mondale to "get tough" with Reagan are pushing on an open door. And they are ignoring the history that suggests that every time an opponent tries to land a haymaker on Reagan's jaw, Reagan slips the punch and the opponent goes sprawling.

Less metaphorically, the point is that if the predictable Mondale attack on the Reagan policies and record is to be politically effective, Mondale must first establish himself as a leader offering a credible alternative to the incumbent.

His leadership credentials are suspect to many voters, partly because of his Carter administration ties, partly because of the attacks on him from his Democratic primary rivals.

His policy ideas are not yet well known, largely because most voters are only now tuning in on the campaign.

The past 10 days have offered grounds for wondering how well Mondale will do in these respects -- particularly in the establishment of his own plausibility as a prospective president.

But he cannot do that job by going out and beating up on Reagan every day. And those Democrats who are urging that course on Mondale miss the mark as much as they misjudge the man at the head of their ticket.