State Sen. Jerome F. Connell Sr., once the powerful chairman of the Maryland Senate's Economic Affairs Committee, was convicted tonight of federal income tax evasion and filing false tax returns.

Connell and his law partner, Lloyd E. Clinton Jr., were found guilty of evading taxes on $75,000 of income to their Glen Burnie law practice during 1979, 1980 and 1981. Government prosecutors said the two lawyers collected many of their client fees in cash and deliberately schemed to avoid paying taxes on the money.

A U.S. District Court jury, which heard testimony from 168 witnesses during the 3 1/2-week trial, found Connell and Clinton guilty on all six counts each was charged with. The verdict was reached about 8:30 p.m. after less than 2 1/2 hours of deliberations.

Connell and Clinton sat stiffly side by side, staring straight ahead, as the verdict was read. Connell shook his head slightly as the jury foreman read out "guilty" to each of the six counts against him.

Connell, 58, a three-term Democrat from Anne Arundel County who sponsored some of the legislation that reduced state regulation of the banking industry, stepped down from his committee chairmanship after his indictment last year. A trial on the same charges ended in a mistrial in the spring of this year after presiding Judge James R. Miller Jr. had a heart attack.

Under the Maryland Constitution, Connell will be suspended from the Senate upon sentencing. If he does not appeal or if his appeal is denied, he will be expelled.

Connell and Clinton each could be sentenced to up to 15 years in prison and fines of up to $30,000. Sentencing was set for Dec. 2.

After the verdict was announced, Connell said on the courthouse steps that he did not know if he would appeal. Asked about his future in the legislature, he said only: "I would never do anything to hurt the Senate." He said he planned to discuss his future with Senate President Melvin A. Steinberg.

When informed of Connell's conviction tonight in Annapolis, where the Senate is meeting in special session to deal with the state's savings and loan crisis, Steinberg said: "I feel very sad for him as a father and a family person. He served his official role with efficiency and competency. This conviction is related to things of a personal nature, not to his public service."

Connell and Clinton maintained that they believed that the cash fees that they collected were being recorded for tax purposes by the law firm's secretaries and bookkeeper. They conceded that they owe the Internal Revenue Service about $40,000, but they said their failure to pay was caused by chaotic office administration and their own limited background in accounting.

Prosecutors disputed those claims during final arguments today, saying that it was "preposterous" for the defendants to assert that they believed that their office help was recording cash fees that they pocketed in courthouse corridors.

"These guys aren't that thick," prosecutor Max Lauten told jurors. Government lawyers said Clinton had prepared tax returns for 400 clients and was aware of reporting requirements. Connell, they noted, was on the board of a savings and loan association and, in his Senate committee post, was responsible for analyzing the state budget every year.

In all, 168 witnesses testified since the trial began Oct. 1, many of them former legal clients who told the court how they paid Connell and Clinton in cash.

Celeste Maxwell, the firm's bookkeeper, testified that she did not record her bosses' cash transactions, assuming that they would report them on their personal income tax statements.

But defense attorneys Paul R. Kramer and T. Joseph Touhey said Maxwell's sloppy bookkeeping methods were largely to blame for their clients' problems. Both sought to emphasize that in relatively few cases -- 140 out of several thousand during the 1979-81 period -- Connell and Clinton failed to record their fees in the firm's "accounts receivable" ledger.

A key element in the government's effort to prove criminal intent to evade taxes was the defendants' representation of a restaurant owner seeking a liquor license in 1979, under what prosecutor Lauten described today as "most irregular and seedy circumstances."

Michael Stavlas testified during the trial that, at the defendants' request, he paid them a $10,000 fee in cash, which was delivered to Connell in a brown paper bag. Tax records show that only $2,000 of the amount was recorded, and Connell and Clinton acknowleged that they split the remaining $8,000 and pocketed it.

Defense attorney Kramer argued that the $8,000 did not have to be recorded on tax records because it was a "contingency fee" for work on the license to be done in the future. He said, however, that the defendants should have set the money aside in an escrow account.

"If it hadn't been earned yet, why did they split it up 50-50 and spend it?" asked Lauten. "They didn't even put it in a bank anywhere. That money fell off the face of the earth."

Stavlas testified that the $8,000 was not returned to him, and he said Connell and Clinton told him that more work on obtaining full liquor licensing would cost an additional $12,000.

Connell is the first member of the Maryland General Assembly to be convicted of a crime since state Sen. Tommie Broadwater (D-Prince George's) lost his seat after being convicted of food stamp fraud two years ago. Broadwater served 4 1/2 months in federal prison in 1984.