On the eve of the Fourth, let us make a unilateral Declaration of Independence from the conventional wisdom of Washington. While ''everyone knows'' that Ronald Reagan is all washed up, let us consider the seemingly remote possibility that he has an opening for another of the remarkable recoveries which have marked his career.

It's risky to go down this road. On Sunday, The New York Times had a front-page headline, ''Reagan Prospects as Leader Dim,'' and CBS did a long television piece on the same theme. They are rarely wrong. But oddly enough, Reagan has just been handed two -- maybe three -- golden opportunities to make them eat their words.

Supreme Court Justice Lewis F. Powell, Jr.'s surprise resignation has opened the way for Reagan to reshape the long-term direction of the high court. The Democratic Congress has passed a budget that invites Reagan to mount his white horse and go into battle against his favorite targets, the tax-raisers. And Mikhail Gorbachev has launched a huge domestic economic gamble that increases the odds for an interim nuclear arms-control agreement and reduces the risk of Soviet foreign military adventures.

Clearly, Reagan's standing has been badly damaged by the Iran-contra affair, and he remains at risk until Oliver North and John Poindexter have said their pieces. But investigating Democratic senators such as Oklahoma's David Boren will tell you that their constituents still are pulling for Reagan to finish his term with his head high. And, contrary to the current Washington theories, the instruments of his recovery are now visible.

The Powell vacancy is sheer bliss for Reagan. The third appointment of his tenure allows him to replace the ''swing man'' on a whole series of social-issue questions crucial to the Reagan constituency. If he finds a nominee with strong personal and legal credentials -- as he certainly can -- he can force the opposition to fight on the ideological ground where Reagan has always felt strongest and most comfortable.

The lines will be very clearly drawn during the Judiciary Committee hearings, with two of the Democratic presidential candidates -- Chairman Joseph R. Biden Jr. and Sen. Paul Simon -- plus Senate Majority Leader Robert C. Byrd and Sen. Edward M. Kennedy all on one side of the committee table, along with two other northern liberals and two moderates. On the Republican side, Deputy Minority Leader Alan Simpson will lead what should be a near-solid lineup in support of the president's choice.

Biden is really under the gun, facing a test he cannot afford to lose. If Reagan sends up a first-rate conservative, Biden can conceivably choose to play the statesman and escort that person to confirmation, while making it clear he does not endorse the nominee's views. But if Biden sets out to defeat the president's choice, he had better line up his votes. He has drawn some criticism for inept generalship on the 1986 Manion nomination. To have a Democratic Senate confirm a Supreme Court justice he had vowed to defeat would leave Biden hanging out to dry.

That's a fight the president will enjoy, as is the battle now joined over spending priorities and taxes. On Friday, Reagan is going to reach back to a 1984 idea that was floated by one of his early braintrusters, Martin Anderson, and unveil an ''Economic Bill of Rights.'' It will include a reformed budget process, a balanced-budget amendment and line-item veto authority for the president.

These are shopworn ideas dressed up in wrapping Anderson designed for the 1984 campaign -- before Reagan decided to do the ''morning in America'' schmaltz instead of talking issues that fall.

What Anderson understands is that Reagan is at his best talking issues -- as long as those issues embody the simple verities he has preached for years, such as the virtues of smaller government and lower taxes. The Democrats have given Reagan his cue by renewing their call for tax increases.

Republican Party leaders meeting last week in New Orleans were enthusiastically of the view that Reagan has the winning side of the debate. The vagueness of the Democrats' tax plans allow Republicans to tell every group of voters they may be the ones who have to pay. That tactic worked very well for Margaret Thatcher in reversing a Labor Party surge in the final week of last month's British election campaign, and Republicans believe that Reagan can make it work here.

As for the Gorbachev opening, the Soviet leader is embarked on a high-risk economic counterrevolution in Russia at least as bold as the Chinese communists are attempting in their land. At the very least, developments in the two communist nations allow Reagan to claim that his market-oriented, individual-enterprise ideas are the wave of the international future. At best, these developments could usher in a period of peace and international cooperation which would permit Reagan to crown his presidency with significant arms-control and trade agreements.

That's a long way from being a lame duck.