A mistrial was declared today in the week-old federal income tax evasion trial of Maryland State Sen. Jerome F. Connell Sr. and his law partner after presiding Judge James R. Miller Jr. was hospitalized Sunday following a heart attack.

The mistrial was ordered by Judge Frank A. Kaufman, chief judge of the Maryland federal court, but he set no specific date for a retrial. Miller was admitted to the intensive care unit of Memorial Hospital in Easton near his Eastern Shore home in Bozman, Md. The 53-year-old judge was listed in serious condition.

Prosecutors had completed three days of testimony last week before Miller and a jury of 10 men and two women in what promised to be a three- to four-week-long trial.

Connell, longtime Anne Arundel County senator, and his Glen Burnie law partner, Lloyd E. Clinton Jr., were charged with evading federal taxes on about $69,000 in client fees, much of it in undocumented cash payments, from 1979 through 1981.

The two defendants have acknowledged that they failed to report some client payments. But they contend that this resulted from a breakdown in office bookkeeping procedures and was not intentional.

In a series of conference telephone calls Sunday night after Miller was hospitalized, Kaufman discussed possible alternatives with prosecutors Robert B. Green and Robert J. Mathias and defense attorneys Paul R. Kramer and T. Joseph Touhey.

The prosecutors reportedly indicated they would agree to the assignment of a new judge to take over the case before the existing jury. But the defense felt, as Kramer said in an interview today, that "things were going well for us before Judge Miller," and asked for a mistrial, hoping it would be reassigned to him.