Patrick Leahy has used up half his savings. Gary Hart has had to struggle to stay even. But George McGovern has built up a comfortable blanket of financial security. And Daniel Inouye's net worth tripled over a 10-year period.

Those examples reflect the disparate results of a survey by The Washington Post of the personal financial implications of service in the U. S. Senate.

The survey, based on senators financial disclosures and studies by various groups, shows that some senators have made considerable financial gains while in office, while some others have experienced relatively tight financial straits - particularly during their first years in Congress.

For the most part, the survey shows that senators - even before receiving the new $12,900 pay raise - are a prosperous group of men who are far better off financially than most of their constituents.

The survey seems to disprove the suggestion, raised in recent months by supporters of the pay raise, that serving in Congress can be a costly drain on a member's financial resources.

The U. S. Senate has been known throughout its history as a rich man's club, and the current membership supports that description. At least 18 senators are millionaires, and about 30 others have net worth great than $250,000.

In contrast, a study last year by Ralph Nader's Citizens' Action Group found only five senators who reported a net worth under $50,000. All were first-term members except Bob Packwood (R-Ore).

Some wealthy senators inherited their money, and some earned it through business interests before entering government. But some have made significant financial progress during their service in Congress.

A few examples:

George McGovern, the South Dakota Democrat who was a history teacher at a small college before entering Congress, told his constituents in 1962 that his successful race for the Senate that year had left him $25,000 in debt.

After 15 years in the Senate, McGovern today reports a net worth of $237,000. He lives in a $135,000 house in northwest Washington, and owns a stately home near Kalorama Circle that he rents out for $3,000 per month. He recently gave his children a $65,000 house in St. Michael's on Maryland's eastern shore.

McGovern's financial growth is due to gains on his Washington real estate and fees for lectures and television appearances. Last year the senator wa paid $25,000 by ABC news for a week's work as a political analyst.

When Hubert H. Humphrey (D-Minn.) came to the Senate in 1949, his wealth totaled about $50,000. By 1965, when he left the Senate to become Vice President, he was worth $171,000. Today, after a term as Vice President, two years of teaching and lecturing and another Senate term, he and his wife report a combined net worth of $637,000.

Humphrey says "the increase in my net worth has resulted from hard work, good investments, frugality, and increased income." He says he earned a "good" income in 1969-70, when he was out of government, and has received income from a writing and lecturing since returning to the Senate.

Sen. Daniel Inouye (D-Hawaii) reported a net worth of $72,000 in 1964-, his first year in the Senate. At the beginning of 1974 he reported his worth as $224,000. The increase came from successful investments in real estate and corporate stocks. Inouye also earned about $13,000 annually in lecture fees in addition to his Senate salary between 1969 and 1973.

Sen. Edward Brooke (R-Mass.) was elected to the Senate in 1966 after a decade of law pracice and five years in state and local government. Since coming to the Senate, he has purchased a vacation retreat. "LaBatterie," including a home and 10-acre estate, in the French West Indies, and a cooperative apartment in the Watergate complex. His equity in real estate is estimated at about $200,000; last year he reported stock holdings worth $95,500.

Brooke says he remortgaged his family home in Massachusetts to purchase the real estate he bought while in the Senate.

At the other end of the spectrum is Sen. James Abourezk (D-S.D.), who would appear to qualify for the title of "poorest senator" on the basis of senators' voluntary financial diclosures.

Abourezk last year listed his net worth at $23,433, down from the $24,858 he reported just after his election in 1972. Those figures do not include "a small amount of property" his wife inherited, the senator says.

Other senators who reported a net worht last year under $50,000 were Dick Clark (D-Iowa), Gary Hard (D-Colo.), John Durkin (D-N.H.) and Packwood.

Generally, those who come to the Senate with limited personal assets encounter financial problems in their first years here.

After one year in office, Abourezk had to borrow $6,000 to pay his federal income tax. Leahy spent so much of his savings buying a home in suburban Washington and settling in here, he says, that his net worth dropped from $95,000 to $52,000 in one year.

"The firsy year here was really hell," Hard recalled in an interview recently.

"I knew it was going to be expensive coming to Washington, but I had hoped we could keep our house in Denver. It turned out that we sold that house and still had to worry a lot about money that year."

Hart said he had been paying a $200 monthly mortgate bill on his Denver home. He sold that house and used teh gain for the downpayment on a house in Bethesda. His new mortgate payment is nearly $800 monthly; in addition, he pays $100 monthly rent on an efficiency apartment in Denver.

Although Hart is reimbursed for transportation costs when he goes home, he does not receive a per diem allowance. Thus, a week's travel through his state can cost him $200 or more for hotel and miscellaneous expenses. (Members of Congress do receive per diem for travel on committee business.)

Although Hart had expected to bear those costs, he was flabbergasted to discover the out-of-pocket expense he bore for social obligations to his constituents.

"You're always sending flowers for a funeral, or wedding presents, or bar mitzvah gifts for the kids of somebody who worked on the campaign," the senator said. "It's nickels and dimes here and there, but you end up spending a couple of thousand a year on it."

If he had more money, Hard said he would like to entertain more econstituents when they visit Washington.

"I see my collegues in the (Senate) restaurant with six, 10 people tey're taking to lunch. It would be nic if I could do that, but I haven't been able to afford it."

Like his senior collegues, Hart graduall found ways to soften his financial predicament. A concert featuring pop singer Linda Ronstadt raised about $5,000 for "constituents' fund" to defray office costs. University and interest groups were willing to pay $500 or more to hear a Gary Hart lecture, and the senator has added about $12,000 annually to his income through speaking engagements.

Even before the new pay raise, Hart had surmounted the serious financial worries of his freshman year.

"I'm not complaining," Hart said. "I didn't run for the Senate to get rich. But I'm a saver. I want to be saving money for college, for the future, and I can't do it."

Whether they grow rich or poor during their years on Capitol Hill, members of Congress generally sacrifice less and profit more from government service than officials in the executive and judicial branches, according to a study financed by the presidential commission of federal salaries.

The study showed that judges average a salary cut of 33 per cent when entering government, and top executive officials average a 23 per cent drop from their private sector salaries. But new members average a 2 per cent salary increase when they enter Congress.

Upon leaving government, top executive officials can expect a new salary 87 per cent higher than their government pay, the study found. Those few judges who leave federal service average an 84 per cent salary gain. Retired or defeated members of Congress can expect salaries 34 per cent higher than their Congressional pay.

Comparing the last private salary earned before entering government to the first salary earned after leaving, congressmen average a 61 per cent net salary gain. Executive officials have a net gain of 44 per cent, and judges 23 per cent.

Because the financial sacrifice, or "opportunity cost," of federal service was found to be smaller for congressmen than for employees of the other branches, the salary commission recommended larger increases for judges and most top exeuctive officials than for congressmen.

But in the new salary scales just adopted, Congress and the judiciary both received raises of about 29 per cent, while executive positions were given smaller raises.

Like other studies of senators' financial status, The Washington Post survey was hindered by a lack of complete and reliable data.

About 20 senators refuse to disclose their public financial condition.

Some who do make public financial statements employ practices - such as valuing their holdings at cost, or excluding property held in their wives' names - which make it difficult to draw an accurate picture.

Some senators who are most outspoken about the need for disclosure provided incomplete disclosures of their own holdings.

Sen. Howard Cannon (D-Nev.) has sponsored legislation that would require all federal employees making $25,000 per year or more "to make full public disclosure of their personal holdings," as the senator describes it.

Cannon released a detailed statement of his own holdings and net worth in 1970. Last year he released a briefer statement, and declined to reveal his net worth.

Former Sen. Hugh Scott (R-Pa.) was a co-sponsor of Cannon's disclosure bill. Scott regularly rebuffed questions about his personal financial holdings, even after allegations that Gulf Oil corp. had paid him tens of thousands of dollars in cash over a period of years.

In 1964, Sen. Harrison A. Williams (D-N.J.( took the Senate floor for an eloquent speech insisting that senators and most Senate staff aides should make public their financial holdings each year.

Williams annually issues a personal financial statement. But he refused to reveal the holdings of his wife, Jeannette Williams, who is one of his chief staff aides.