Mr. Inside-Outside...

IN his press conference Wednesday night, President Reagan reminded the public that he is running for office again - this time in the dual role of White House incumbent and Washington outsider.

This ability to campaign as an assertive leader of the government, but at the same time a bold critic outside it - positioned between the people and their government - is one of this President's unique traits.

How it works for him in 1984 will depend partly on how well he tags Congress as the ''government'' responsible for the Washington ''mess,'' and how successfully he links the White House with economic recovery and other Washington pluses of the past 31/2 years.

It's up to the public, of course, to assess credit and blame. The public tends to think any criticism of the White House undercuts a president's effectiveness. And it is more apt to blame Congress than the White House when things go wrong.

Running against Congress has worked well for previous presidents. In 1948 Harry S. Truman successfully ran for reelection largely by railing against the ''do-nothing 80th Congress.'' Eight years earlier Franklin Delano Roosevelt had campaigned, in part, against a Congress too conservative for his liking.

In any case, the current Congress's alleged obstructionism has not hurt Mr. Reagan's popularity. Neither have the accusations of the Democratic presidential candidates. His approval rating continues to float in the safe 50 percent-plus range.

Ironically, for the White House as well as Congress, 1984 could be a good year for incumbents, whatever the level of blame-shoveling up and down Pennsylvania Avenue.