Along with most of the media and the politicians here, President Reagan's men view the stock market plunge with bemused incomprehension -- failing to perceive that its root is deep in their own policies.

The start of deep and alarming fluctuations on Wall Street coincided with signals two months ago from the Santa Barbara White House that the Reagan Revolution had entered its Thermidor. It is no coincidence that the market's fearsome drops came after Reagan demonstrated his inability to confirm a Supreme Court justice.

The market, a highly disciplined barometer tried and tested by history, clearly perceives an uncertain trumpet in Washington. It hears no true signal that Reagan will challenge an obstreperous Democratic Congress by vetoing trade, tax and spending legislation -- and go to the mat to uphold those vetoes.

While Howard Baker's White House staff sees no link between Bork and stock prices, the connection is obvious to the administration's surviving Reaganite true believers. ''I think the markets see the president as the last barrier against higher taxes, against protectionism, against inflation, against higher spending,'' one such official told us.

Ronald Reagan's uncertain trumpet sounded from California two months ago. Market turbulence can be traced to the Aug. 23 New York Times dispatch from Santa Barbara by Steven V. Roberts reporting that Reagan's advisers had ''decided the White House would do better avoiding vetoes and reaching a compromise with Congress'' on such key issues as trade. That faithfully reflected what senior staffers had been saying in Washington.

The implication Reagan would sign any trade bill rather than suffer an override struck a discordant note in world markets and at the Federal Reserve Board. Fed Vice Chairman Manuel Johnson let it be known that a signed trade bill would be destabilizing enough to force severe tightening by the central bank, aborting U.S. economic growth. Since then, Reagan administration officials led by Treasury Secretary James A. Baker III have come over harder in predicting a trade veto. But then the Bork fiasco raised new credibility questions about a Reagan veto and his capacity to sustain it.

When Sen. Howell Heflin of Alabama announced his opposition on Oct. 6, signaling the markets that conservative southerners were deserting Reagan, the Dow Jones fell almost 50 points in an hour. Since then, White House management of the Bork disaster has coincided with market unease.

The substitution of softer rhetoric in the president's New Jersey speech last week is explained at the White House as resulting from Bork's request that speeches in his behalf follow his own tone. Burying Reagan's oft-postponed defense of Bork in a non-televised afternoon speech is blamed on the networks, which made clear they would not run it in prime time either. Nevertheless, when NBC followed the course of CBS and ABC in declining to carry the speech, stock prices plunged. Declining credibility of any president, whether Jimmy Carter or Ronald Reagan, often affects markets adversely.

The announcement Wednesday of a trade deficit slightly higher than expected set off a fantastic three-day drop in the Dow, around 250 points, based on fear. Investors feared not a continuing trade imbalance but what a protectionist Congress, unimpeded by a strong president, would do about it.

This is widely understood in the real world, where market discipline is routinely accepted. But when the link between Washington politics and the markets was suggested at a senior White House staff meeting several weeks ago, there was no response. In fact, presidential staffers dismiss the Bork affair as a cause of plunging stock prices, which they attribute solely to international currency fluctuations.

While his own men seem ignorant about political weakness as a market destabilizer, Reagan himself hinted last week that he instinctively understands the limits of accommodation. Meeting with conservative Republican senators last Thursday, the president departed from his index cards to say it sometimes is necessary to lose in a good cause. That sentiment, if carried into action, might do more to help Reagan's credibility in the world of finance than the sequence of retreats for which the markets are bracing.