Bert Lance yesterday defended himself against many - but not all - of the questions raised in recent weeks about his banking career and his confirmation as director of the Office of Management and Budget.

Lance took the occasion of his "day of court" to go on the offensive. He gave no hint that he is considering resignation at the end of the Senate hearings now being conducted. "My conscience is clear," he said.

"I did not ask for this fight, but now that I am in it, I am fighting not only for myself and for my family, but also for our system," Lance declared.

He said he did not hide unfavorable information about himself from the Senate at the time of his confirmation, and he repeatedly denied that his past banking practices were unethical or illegal.

"I do not contend that I made no mistakes" as a banker, Lance said, but he admitted none. And he proudly described the growth of the two Georgia banks he ran. He defended the large overdrafts permitted to him and his family by the Calhoun First National Bank as good banking policy designed to win friends - and thus business - for that bank.

The scene yesterday morning as Lance began his attempt at self-vindication before the Senate Governmental Affairs Committee was fit for a courtroom drama. Judging by the statements of committee members in recent days, a jovial, rotund Christian - Lance - seemed headed for an angry pride of hungry lions.

But it turned out differently. From the moment he arrived in the hearing room, clasping the hand of his wife, LaBelle, Lance exuded both his familiar good nature and unfailing self-confidence. He had one awkward moment trying to cope with a hard question from Sen. Charles H. Percy (R-Ill), but generally cruished through the day without deviating from a long, carefully drafted opening statement that dealt with most of the questions that have been raised about him.

Because of the length of the statement - which Lance read in a calm voice for nearly two hours while his lawyer, Clark M. Clifford, mouthed the words silently beside him - and because of extended haggling among committee members, Lance faced relatively little interrogation yesterday. The hearings will continue today and probably on Saturday.

The principal lines of Lance's defense were: whatever the comptroller of the currency or other critics might say of his banking practices, no one ever accused Lance of violating the law and no one "ever lost a cent" in the two banks he ran.

Moreover, he has always repaid his loans, as have his wife and members of her family. They all repaid the overdrafts from the Calhoun bank, too.

In other words, as a practical matter, everything turned out fine, even if there was some inattention to detail. And that happened Lance said, because he had other obligations - "I headed the (Georgia) state highway department from 1971-73, and then from October, 1973 to December, 1974, I was campaigning" for governor, and in 1975 he joined the National Bank of Georgia in Atlanta as its president.

As for misleading the Senate, Lance simply denied it. At a meeting with members of the staff of the Senate committee in January before his confirmation hearing, Lance declared, "I disclosed . . . the various financial matters which now are the focus of this hearing."

According to Lance, he told the committee staff about the bank overdrafts that helped finance his 1974 gubernatorial campaign, about the Justice Department investigation of them, and about the Calhoun bank's troubles with the comptroller's bank examiners, including the cease-and-desists agreement imposed on the bank in 1975, and "I advised them of my personal overdrafts."

This contradicts previous statements by members of the committee and staff who have claimed incomplete advance knowledge about many of these matters. Sen. Jacob K. Javits (R-N.Y.) said last night that the staff members who interviewed Lance will be put on the stand today to testify on what they remember from the January meeting.

Lance expressed indignation at the publicity his case has received and said his human rights had been violated:

"The rights that I thought I had as an American have been treated in the most irresponsible and destructive manner. The basic American principle of justice and fair play has been pointedly ignored by certain members of this committee. Certain persons have publicly, in effect, brought in a verdict of 'guilty' before I have been given the opportunity to present my side of the case."

Lance's most difficult moment yesterday came in response to questioning from Percy about the allegation that Lance simultaneously pledged the same bank stock as collateral for two different loans from two New York banks.

Lance signed a note from the Manufacturers Hanover Trust Co. for a loan of $2,625,000 in June, 1975. He posted 148,118 shares of stock in the National Bank of Georgia as part of the collateral.

The standard note agreement he signed included the requirement that he "forthwith deliver . . . to the bank" any stock dividend he might receive on the share already posted as collateral.

Lance subsequently received a stock dividend of nearly 15,000 additional shares of NBG stock, but he did not "forthwith deliver" it to tha Manufacturers Hanover bank. Instead, he pledged this stock to the Chemical Bank of New York as collateral on a $150,000 loan.

Percy tried repeatedly to get Lance to admit that this sequence of events occurred (as the comptroller of the currency's report on Lance shows) and to elicit an acknowledgement that it might have been improper.

But Lance insisted that he was "negotiating" with the Manufacturers Hanover bank about delivering this stock because he believed his original loan was fully collateralized regardless, in effect, of what the note he signed said.

"Clifford, his lawyer, added that Manufacturers Hanover's claim on that stock dividend was discretionary - the bank had to ask for it to get it. Percy noted the evidence in the record that the bank had asked repeatedly for it.

At one point Perry said Lance had obviously read the Manufacturers Hanover note that he signed, but Lance interrupted him. "No sir, I did not read the note," he said.

(Percy did not note that in a July 1, 1976, proxy statement to NBG stockholders the bank reported that Lance had "pledged 162,929 of the NBG shares held by them" to Manufacturers Hanover as collateral for a loan.

(That amount of stock would have been the original collateral plus the stock dividend of nearly 15,000 shares. Despite the assertion in the proxy statement, however, this amount never was pledged to Manufacturers Hanover. Instead, 14,811 shares of it was pledged and delivered to Chemical Bank for the other, $150,000 loan.

(Reporters only discovered this reference in the proxy statement last night.)

Lance also was questioned by committee Chairman Abraham A. Ribicoff (D-Conn.), Sen. Henry M. Jackson (D-Wash.) and Javits about his and his relatives' large overdrafts. Jackson said he thought this was the crucial issue. "The average man in the street has a deep concern about a course of conduct here," he said.

Lance said that overdrafts were just a means of extending credit to a bank customer and, "even though it's a practice that was critized" by federal bank examiners, he regarded it as a useful policy to win friends and customers for the bank.

Javits challenged this notion by asking if the bank needed to court its officers and owners - Lance and his in-laws - by allowing them overdrafts that totaled hundreds of thousands of dollars.

"You weren't a new customer," Javits said. "Nobody had to encourage or solicit your business. Your were the chief executive officer."

Lance replied that his accounts were very profitable to the bank because he maintained large cash balances. Therefore, he implied, the overdrafts he had were not unwarranted.

And he said his wife and her relatives (whose joint overdraft at the Calhoun bank at one point totaled more than $450,000) were wealthy people who always had more than enough assets to cover their obligaitons and overdrafts.

In his prepared statement, Lance said he made "no apology" for the overdraft policy at the Calhoun bank, a policy he called "liberal." He said he and his family did not enjoy special privileges, that the liberal policy "was available to all depositors."

He said that the fact that the bank lost very little money because of overdrafts - about $28,000 in five years, he said - proved that this was not an unsafe and unsound practice.

And he noted that once the comptroller's office ordered the bank to stop allowing big long-term overdrafts, the bank complied.

Official sources have predicted that Lance's biggest potential legal problem may involve his use of planes belonging to the Georgia banks he ran. On this issue, he took the position that it would be "impossible" to neatly distinguish business trips from private trips.

Regarding the NBG plane, Lance said: "No specific guidelines were established by the bank for the use of the plane. It was generally understood that I was to be creating a new image for the bank and would be seeking new business whenever and wherever I traveled. While one might not be able to connect a particular plane trip to a specific piece of new business that was obtained as a result, it was understood that the achievement of the bank's growth objective would depend largely on the new image we could create for the bank. This required extensive plane travel."

(The Washington Post has quoted sources as saying Lance used the plane on family vacations to fly his children to school and for political trips. The Los Angeles Times reported that the plane had carried such people as President Carter, Cyrus R. Vance, W. Michael Blumenthal and others close to the new administration.

(The Times said Lance has labeled as a business trip a flight that carried Georgia politicians to the 1976 Democratic National Convention and another when a journalist was flown to Atlanta to interview Charles Kirbo, Carter's longtime confident.)

Perhaps Lance's most ringing self-defense was offered in connection with Billy Lee Campbell, the former officer to the Calhoun bank who was found to have embezzled hundreds of thousands of dollars.

Campbell, who is serving a prison term for embezzlement told Senate investigators last month that Lance could be implicated in his crime, according to authoritative sources.

Numerous lawyers involved in the case have strongly denied that Lance had any connection with Campbell, a message Lance reiterated yesterday. Ribicoff noted that he and other committee members had put little credence in Campbell's reported testimony.

Lance also forcefully refuted the suggestion - advanced last week by Percy - that he might have deliberately backdated three checks early this year to falsely calm a tax deduction. Lance outlined a sequence of events showing what had happened - and noted that his 1976 tax return showed no tax deductions of the sort that the backdating would have made possible.