THANKS TO PRESSURE from 13 Republican senators, President Reagan has dropped his support of a nominee whom he had officially selected for a District of Columbia judgeship. This is no routine withdrawal of a federal nomination in the interest of intraparty harmony. Mr. Reagan has caved in to political intervention in what should be a purely local matter. The result is a reversal of the important progress toward more self-government that was envisioned in the judicial selection process approved back in the Nixon years.

It's a also a disgrace in terms of filling an important seat on the local bench. Last July, Mr. Reagan nominated Deputy U.S. Solicitor General Andrew L. Frey for a judgeship on the D.C. Court of Appeals. As called for by the D.C. charter, the presidehose Mr. Frey from a list of three candidates submitted by a seven-member D.C. Judicial Nominations Commission.

Enter the 13 senators -- including Paul S. Trible of Virginia -- with a letter to the president noting that they would have "no choice but to oppose" the nomination because Mr. Frey's affiliations do not concur with party policy. The senators went on to advise Mr. Reagan that Mr. Frey's "political and judicial philosophy is very much at odds with your own philosophy and the public positions you have taken."

Now -- eight months later -- the White House announces that Mr. Reagan intends to nominate another of the candidates, John Montague Steadman, even though Mr. Frey said on Friday that he himself had not withdrawn. Technically, of course, the president is still choosing from the locally submitted list. But why should national ideological standards set by a group of senators be enough to kill a nomination for a purely local judgeship?

Neither President Reagan nor the full Senate should let this group get away with exerting this sort of pressure on a local issue. Their failure to resist is a strong argument for taking a further step to improve local self-government here: delegate the nomination of D.C. judges to the elected mayor and council. Hometown selection of local judges is the American way in state and local courts everywhere else in this land.