Does the Hatch Act bar federal employees from engaging in "political activity"?

In CSC v. Letter Carriers, 413 U.S. 548, 557, 93 S.Ct. 2880, 2886, 37 L.Ed.2d 796, the court said: ". . . it is in the best interest of the country, indeed essential, that federal service should depend upon meritorious performance rather than political service, and that the political influence of federal employees on others and on the electoral process should be limited." The court thus recognized that interest as important enough to prohibit "political activity" by federal employees under the Hatch Act, even though such prohibition to that extent limited the exercise of free speech.