Blast From Windy City: National political leaders still are trying to read the seismographs on the Chicago mayoral race. But there is bipartisan agreement that while Mayor Jane M. Byrne's write-in campaign could help Rep. Harold Washington (D-Ill.), the black Democratic nominee, by splitting the white vote, it could be bad for both parties. It would be even worse if he loses, unlikely as that seems.

Democrats fear that if Washington loses the bulk of the white vote, as does seem likely, to Bernard Epton, the Republican nominee, and Byrne, blacks may conclude that they cannot get a fair shake in the party.

Thus, even though they are the most loyal element of the Democratic coalition, they will sit on their hands nationwide in 1984. Republicans fear that however it turns out, blacks will view Epton's effort as a racist campaign, jeopardizing their hopes of somehow increasing their slim share of the black vote.

There is other fallout. The first reaction by some national Democratic spokesmen was that Washington's nomination and probable election helped Chicago's bid for the 1984 Democratic National Convention. National Chairman Charles T. Manatt is from California, and San Francisco had been considered the favorite. With Byrne's amazing reentry as a write-in, however, national party leaders say that financial and political support in Chicago for the convention has begun to diminish.

Another ripple: national party sources say that David Sawyer, the New York television expert who did Byrne's primary campaign ads, is being punished for running her write-in campaign. Sources say the DNC has removed his name from the list of consultants given to Democratic candidates and that he cannot expect to get future contracts with the national party such as the one he had for the response to President Reagan's State of the Union speech.