The Senate Rules Committee is expected to approve today President Carter's nomination of John McGarry to a Democratic seat on the Federal Election Commission.

McGarry had faced tough questioning by Republican committee members about his income tax returns and financial disclosure statements filed with the House of Representatives. But yesterday, after studying additional information from the Internal Revenue Service. Republican staff members decided there was no need for further hearings and supported a committee vote today as scheduled.

"There is a time when you have to draw an investigation to an end," a Republican staff aide said. "The facts are now on paper. We didn't find anything (in the IRS documents) that we felt needed further discussion."

Before voting today, the committee will consider a report on McGarry's finances and the IRS audits compiled by Reka Hoff, a General Accounting Office tax expert assisting the Rules Committee.

In earlier hearings, Hoff had charged that McGarry took improper deductions and made miscalculations on certain recent income tax returns.

But she stressed that she needed more information from the IRS to be certain of any wrongdoing.

On Wednesday, McGarry authorized the IRS to release additional information to Hoff and the committee.

That information showed that full audits of McGarry's income tax returns were conducted in 1973 and 1974, and that briefer audits had been made for 1971, 1972, 1975 and 1976.

In particular, IRS auditors had examined and approved a computer leasing arrangement and at least some of the commuting expenses Hoff had identified as improperly computed by McGarry.

Sen. Mark Hatfield (R-Ore.), who lead the questioning of McGarry, had insisted the committee should get answers for the questions raised by Hoff before any vote.

McGarry is a special counsel to the House Administration Committee and a friend of House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) He was first nominated to the FEC last September, but the Senate failed to act and the nomination lapsed at the end of the session.

Carter resubmitted McGarry's name in April. In addition to facing extensive staff inquiries, McGarry had appeared before the Rules Committee for questioning four times in the past four weeks.