It has become the vogue among football writers trying to explain the imponderables of what used to be called the home-team advantage to personalize it as "the 12th man."

As President Reagan formally launches his second term, he also has a "12th man" on his side, one who might be called Persistent Underestimation.

Old P.U. has had quite a run with Reagan. In 1966, in his first campaign, Reagan was compared unfavorably with Bonzo, a chimpanzee who had upstaged him in the movie "Bedtime for Bonzo."

The man who did the underestimating then was California Gov. Edmund G. (Pat) Brown, who had the excuse of never having seen Reagan campaign. Subsequently, Reagan was underestimated by California's dominant Democratic politician of the day, State Assembly Speaker Jesse Unruh, who in that regard proved a forerunner of House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.).

The underestimation of Reagan has been bipartisan. Republican legislators in Sacramento were more contemptuous of him than Unruh was, and Gerald R. Ford refused to take Reagan seriously until it was almost too late.

In the fall of 1975, an otherwise brainy Ford aide asked me whether Reagan had the nerve to challenge a sitting GOP president, several months after Reagan had formed a committee to defeat Ford that was "exploratory" in name only.

During the 1980 Iowa caucuses, one of President Jimmy Carter's operatives expressed dismay that George Bush was on his way to victory there. The Carter aide feared that Bush would be the GOP nominee instead of Reagan.

Those of us in the news media have underestimated Reagan even more than the politicians have. Brown's view in 1966 of Reagan's vulnerability was widely shared in the press corps. Four years ago, many of us confidently predicted that Reagan's tax-reduction plan was doomed in the Democratic House. Three years ago, I believed -- and wrote -- that the impact of the Reagan recession would cause him not to seek a second term.

Now here we go again. We have examined Reagan's deficit-reduction plan and found it insufficient. We wonder how the White House will function with a new chief of staff.

In Congress, even the strongest hawks have taken aim at Reagan's bloated defense budget. The orphan MX nuclear missile, abandoned by its friends, faces a March burial. And it is doubtful if even the Great Communicator can resurrect U.S. aid for anti-Sandinista rebels in Nicaragua.

As many now see it, absent a major arms-control agreement, the best Reagan can hope for is a draw with Congress if the economy holds steady in 1985, followed by Democratic capture of the Senate in 1986 and two years of semi-retirement marked by frequent trips to his ranch.

Maybe that is the way it will turn out. But in case it doesn't, here are a few thoughts on why we so often underestimate a man who ranks as one of the century's most accomplished political leaders.

The first thought is that we don't take Reagan seriously because he isn't a politician in the accepted sense. He had a successful career before he entered politics and is uninterested in political details.

This is an asset that politicians tend to represent as a liability. It enables Reagan to think in popular terms and be taken seriously by the many Americans who distrust political solutions. Reagan is never dragged down by details because he is never involved in them. He is not held accountable for his personnel disasters because he is disengaged from his subordinates.

Usually, Reagan keeps his eye on the ball. He has a sense of what he wants to achieve and where he wants to come out, both in negotiating and in historical terms. He also is willing to put aside his ideology in order to save it.

This security in terms of governance makes it possible for Reagan to ignore his advisers and economists. It was Reagan, not his inner circle, who refused to compromise on income-tax reduction or defense in the first term and won because of what was then perceived as stubbornness.

The underestimation of Reagan feeds on itself. It is so easy to compile "Reaganisms" that leave one gasping, so easy to demonstrate gaps in the president's knowledge that expectations for Reagan are set extremely low. This makes a moderate success seem an extraordinary triumph.

Reagan understands this, just as he understood four years ago that there was plenty of power left in a presidency supposedly undermined by Watergate and Vietnam. Isn't it time we stopped underestimating him? Reaganism of the Week:

At an unveiling of photographic portraits of the powerful at the Corcoran Gallery last Monday, Reagan said, "Consider finally the portraits of two skilled and dedicated government servants. Although they have hectic schedules, both are nevertheless looking into the camera calmly, even perhaps with a twinkle in their eyes. Secretary of the Treasury Donald Regan and chief of staff James Baker -- or is it the other way around?"