In Washington, Kathleen O'Reilly and Mark Green won fame over the last decade as sharp and relentless thorns in the backsides of the mighty, and as eloquent champions of the underdog.

This year, the two former consumer activists have left their Washington power base to run for Congress in Michigan and New York. Now, as candidates, they are the underdogs.

Combat medals from the Washington establishment are proving difficult to translate into hard cash and other support necessary to win in the trenches of, say, Washtenaw County, Mich., in 1980.

"It doesn't translate into money, that's a pity," said O'Reilly, 34, former executive director of the Consumer Federation of America, the nation's largest consumer organization. She is challenging moderate Republican Carl Pursell, 47, a two-term incumbent from Michigan's 2nd District. Some folks thought that, with her skill and energy, she had a good chance to knock him off. Now, the Democratic Party's campaign watchers consider her a definite longshot.

Her appearance on televised talk shows earlier this year and last had lit up the switchboards with callers urging her to run. A number of organizations had encouraged her and pledged their support, but then "the minute I announced, they were nowhere to be seen. I know it happens to candidates, but you like to think it won't happen to you. . . . It is unnerving."

Both O'Reilly and Green say their campaign experiences have strongly reinforced their belief that congressional races should be publicly financed the same way that presidential campaigns are now.

Besides being outfinanced, both O'Reilly and Green, who is running in Manhattan's Silk Stocking District, are being tagged by their opponents as carpetbaggers, outsiders from Washington. Both in turn are, not surprisingly, zeroing in on their opponents' records on consumer issues, such as programs for the elderly and health care costs.

In the Michigan race, O'Reilly and Pursell are competing for a vote which has been termed "schizophrenic." The district contains both the largest student population of any in the country (including the University of Michigan at Ann Arbor, a liberal stronghold) and, thanks to the aging auto plants and other industry in the area, the largest population of bowlers, according to O'Reilly.

Pursell, by paying attention to constituent services and keeping his record in the middle of the road, has pleased enough people in this stew of white and blue collar to be elected to two terms, winning with 68 percent of the vote in 1978.

Pursell plans to spend $100,000, as he has in his last two races. O'Reilly hopes to spend that much, but an aide said she has raised only $56,000 so far. (The United Auto Workers, unlike some of her presumed supporters, have contributed the maximum $10,000, an aide noted.) She blames her money problems at least in part on her late start. Tied up with her CFA duties, it was not until April that she decided to move to her parents' home in the district and make the race.

While it is true that O'Reilly had never lived in the district, she grew up in Dearborn, just across the line, according to her aide, Michael Podhorzer, and shares the concerns of the area's citizens. Her uncle is the mayor of Dearborn, he noted, and her cousin is the state's Republican lieutenant governor.

Despite all this, plus numerous television appearances in the area, she said that, as she patrols the district's bowling alleys and laundromats in search of support, she has a problem getting recognized. "A lot of people just hadn't made the link-up between my name and that woman they'd seen on TV. . . . One problem is that I don't look in person like I do on TV." t

O'Reilly contends that Pursell has supported measures, such as energy price decontrol, that increase the burden of inflation on the elderly and those least able to cope with it. And despite his much-heralded position as the only Michigan Republican on the House Appropriations Committee, she says "Virtually every state in the country gets more federal money back than this one."

Pursell is focusing on the problems of inflation and unemployment with a moderate Republican prescription, including a balanced federal budget and "moderate" tax cuts which he says the voters care more about than more narrowly defined consumer issues.

O'Reilly is counting on fund-raising appearances this week by Joan Mondale and Sen. Edward M. Kennedy (D-Mass.) to help narrow the money gap and, she says, she is optimistic. "I'm not disillusioned. Being 100 percent Irish helps."

On the fashionable East Side of Manhattan, a nest of trendsetters and the rich and powerful who control New York's media and publishing centers, Mark Green is waging a similar battle against a Republican incumbent.

The former director of Congress Watch, a chief lieutenant of Ralph Nader and author of numerous books and articles, moved into the district about a month before he declared his challenge. He was born in Brooklyn, according to an aide; his wife is from the district and his family lives here now.

This race, which could be called the Greening of Manhattan, has the added element of confusion caused by the fact that the incumbent is also named Green -- Bill Green. With this extra twist to divert voters, Democratic Party professionals had expected the Democratic Green to run a close, tough race. But now, based on the money he has been able to raise so far, they are calling him a "probable longshot."

The Democratic Green is charging his opponent with posing as a consumer advocate by advertising his progressive stands on such issues as the Equal Rights Amendment and abortion, but concealing his conservative record on broader economic issues and his support from big oil and big business.

The Republican Green maintains he is a moderate on economic issues, argues that his record will get him reelected and, according to an aide, that Mark Green's own Congress Watch gave him a high (93 percent) rating on consumer protection legislation it targeted last year.