The income tax evasion trial of Maryland Sen. Jerome F. Connell Sr., the once-powerful chairman of the legislature's Economic Affairs Committee, began in federal court here today with selection of a jury of 10 men and two women.

Opening arguments by prosecutors and defense attorneys are scheduled to begin Tuesday morning and the trial is expected to last a month.

Connell, an Anne Arundel County Democrat, and his law partner, Lloyd E. Clinton Jr., are charged with failing to report $69,000 in income generated by their law firm in 1979, 1980 and 1981.

According to the indictment and other court papers, the two men reported a combined total of $328,809 in taxable income during the three-year period but failed to report an additional $69,000 -- some of it cash, as distinct from checks and other documented payments -- from clients at their Glen Burnie law offices.

Prosecutors say they have lined up more than 100 witnesses for the trial before U.S. District Judge James R. Miller Jr. Many of the witnesses are said to be clients and former clients of Connell and Clinton.

Defense attorneys have acknowledged that some receipts were not recorded but contend this resulted from a breakdown in office bookkeeping procedures and was not intentional.

Connell, 56, a veteran political figure who has served his north Anne Arundel County district in the General Assembly since 1975, relinquished chairmanship of the key Senate committee shortly after his indictment last summer but kept his seat through the 90-day legislative session.

If convicted, Connell faces a maximum penalty of 24 years in prison and $45,000 in fines plus the costs of prosecution. His indictment marks the second federal prosecution of a Maryland state senator in two years. Former senator Tommie Broadwater Jr., a Democrat from Prince George's County, was convicted of food stamp fraud in 1983 and served four months in prison. CAPTION: Picture, Sen. Jerome F. Connell Sr . . . $69,000 at issue.