There is a billboard above the gritty skyline of Decatur that tells why maverick Republican congressman Paul Findley, now seeking his 12th term, is in a close race this year.

"In Macon County, unemployment: 18.4 percent," reads the sign put up by the local Democratic Party. "It's not fair; it's Republican."

It is also out of date. Since the sign went up a month ago, the jobless rate in Decatur has climbed to just a fraction shy of a staggering 20 percent.

But there are other reasons for Findley's troubles this year. One is redistricting, which changed the 20th Congressional District from one with a slight Republican tilt to one with a Democratic edge, and left him with 35 percent brand-new constituents.

Another is that his opponent, Springfield attorney Richard J. Durbin, served more than 10 years as the state senate parliamentarian, and has used those contacts to make himself the best-organized Democratic House candidate in downstate Illinois. He expects to raise more than $500,000.

Findley, a critic of Israel labeled by detractors as "Yasser Arafat's best friend in Congress," has been targeted for defeat by this country's politically formidable Jewish community and other pro-Israel groups.

Several years ago Findley, a member of the House Foreign Affairs Committee, met with Arafat, leader of the Palestine Liberation Organization, and returned home to advocate direct U.S. negotiations with the PLO. He hasn't backed off since.

"Arafat is, I think, a very practical man who can adjust to reality if it is the only way to get a Palestinian homeland under way," he told a class at Sangamon State College two weeks ago.

That, and Findley's repeated calls for a halt to U.S. aid to Israel, made him "a kind of symbol of a pro-PLO position," said Marvin Josephson, a spokesman for National PAC, a pro-Israel group that gave Durbin $5,000.

Yet Arafat is not the issue he was two years ago, when Findley's opponents -- first in the primary, then in the general election -- ran pictures of the white-haired congressman standing next to the kaffiyeh-clad Palestinian.

The pressing issue today is the economy and the low price of corn. In the district's urban centers, laid-off workers seem prepared to take out their frustration on Republicans.

"I turned coat last time, but I'm never going to do it again. I'm voting straight Democrat," vowed Ed Neally, a worker at an idled automotive plant in Decatur, which was added to Findley's territory by redistricting.

Findley was not an early Reagan supporter and has parted company with the president on the Soviet pipeline sanctions, which have cost jobs at the local Caterpillar and Fiat-Allis plants. When President Reagan landed in Springfield Wednesday to campaign for House Minority Leader Robert H. Michel, Findley was nowhere in sight.

Durbin, an articulate candidate with strong labor backing, has attacked Findley's votes on the budget anyway, labeling him "a loyal foot-soldier" of the Reagan program.

Durbin said he has mentioned the Middle East only twice: once to criticize Findley and once to criticize Israeli Prime Minister Menachem Begin for stalling on the investigation of the Beirut massacre. But he conceded it has been key to his fund-raising.

"He created this issue, I didn't," Durbin said of Findley. "He is totally out of step on the issue. What President Reagan recognizes--what everybody but Paul Findley recognizes--is that the PLO is a force for instability in that region, not a liberation group."

Findley's staff has reprinted copies of Durbin's finance reports to emphasize the out-of-district addresses of his contributors. "Why should people from New York, Miami and Los Angeles decide who is going to represent central Illinois?" asked Don Norton, Findley's campaign manager.

Findley, who expects to spend about $620,000, has solicited contributions from Arab Americans. "What we want in the Middle East is a balanced view," Peter Tanous, chairman of the National Arab American Association, said approvingly.

A penchant for speaking out has made Findley one of Congress' most unorthodox members. A conservative on economic issues, he finds himself aligned with liberal Democrats on foreign policy.

Findley is convinced that his troubles can be traced to his views on the Middle East. "If I hadn't been a persistent critic of Menachem Begin, I wouldn't have had a real contest this year," he said.

Durbin disagreed. "Findley has been saddled with an economic theory that has been a disaster for this district. That's where he's going to lose the election," he said.