John J. Moorcones, a prominent Leesburg lawyer and former Democratic Party activist, tearfully pleaded guilty yesterday in federal court to failure to file a 1976 income tax return.

Moorcones, 42, an unsuccessful candidate for the Virginia House of Delegates in 1965, faces up to one year in prison and a $10,000 fine. District Judge Albert V. Bryan Jr. set sentencing for Dec. 4 in Alexandria.

Moorcones' voice broke as he stood before Bryan to enter his plea and he wept openly as he left the courtroom.

The Internal Revenue Service charged Moorcones, who has considerable Loudoun County business holdings, failed to file a personal tax return covering $81,193 in gross income in 1976.

In return for his guilty plea, federal prosecutors agreed not to press additional charges that Moorcones also failed to file personal returns for 1975 and 1977 and did not file quarterly tax returns covering several employes between 1976 and 1978.

An IRS spokesman said yesterday Moorcones' earnings in the two years omitted in the plea agreement with prosecutors were "substantial."

Moorcones served as assistant campaign director in 1966 for Democrat George P. Rawlings Jr., then of Fredericksburg, in Rawlings' upset primary victory over the late Rep. Howard Smith.