When Brian Berry, a software test manager from Lake Forest Park, decided in May to run for Congress, he thought he had a good shot at making it through at least the primary.

The race he had in mind — a special election in the 1st Congressional District to fill the seat vacated when Democratic Rep. Jay Inslee resigned to run for governor — didn't seem too competitive. Only one other candidate was running when Berry filed on May 15.

But the sleepy race to fill the last month of Inslee's term has turned into the most crowded field on Tuesday's primary ballot — the only one to crack double digits. By the May 18 filing deadline, 11 candidates had forked over the $1,740 filing fee for the chance to sit in Congress for the month of December.

"I was shocked," Berry said.

Five of the nine candidates who dived into the race in the couple of days before the filing deadline are also running for the full two-year term that starts in January.

One of them, Darcy Burner, a Democrat and former political activist, said she chose to run in the special election as well because "it's become increasingly clear that there are some really important issues that are going to come up in December" — the one-two punch of automatic spending cuts and de facto tax increases known as the "fiscal cliff."

The result is a strange contest: a race with 11 candidates, but not much campaigning, for a one-month term during which Congress likely will deal with hugely important issues.

"This is a highly unusual situation we're in, I can tell you that," said David Ammons, a spokesman for the Washington Secretary of State's Office, which oversees elections.

It gets stranger. The race for the full term is taking place in the new 1st Congressional District, which, because of redistricting, stretches from Medina up to British Columbia.

But the contest to fill the last month of Inslee's term is being fought in the old 1st District that elected him in 2010, which takes in Seattle's northern suburbs and much of Kitsap County and tilts more Democratic. That means voters in Edmonds and Shoreline, for instance, are casting ballots in the old 1st District for the one-month term and in the 7th District, in which the towns now fall thanks to redistricting, for the next session of Congress.

"I think that is without precedent," Ammons said.

Other parts of the old 1st District now fall in the 2nd and 6th Congressional districts. And the different political leanings of the old 1st District and the new one may make it more likely that voters in the two districts will select different winners on Tuesday.

The situation has prompted a number of questions from voters. "It's very confusing for people," said John Koster, a Republican who is also running for the full term. "I can't tell you how many calls I've had and made an attempt to explain it to people."

When the state announced it would hold a primary to fill Inslee's seat, the Washington State Democrats attempted to rally support around Brian Sullivan, a member of the Snohomish County Council, as a consensus candidate.

But there were five Democrats running for the full two-year term in the 1st District — which will appear on the primary ballot on Tuesday right above the race to fill the last month of Inslee's term — and not all of them were willing to cede the seat to Sullivan.

"Dwight Pelz attempted to bully everyone in the race into accepting his candidate," Burner said, referring to the chairman of the Washington State Democrats.

Burner filed to run at 9:28 a.m. on May 18, less than eight hours before the deadline. By day's end, six other candidates had leapt into the race, including three other Democrats running for the full term: Laura Ruderman, Suzan DelBene and Darshan Rauniyar. Koster, the Republican candidate for the full term, also filed on May 18.

"Once Darcy jumped in, the dominoes all fell," Koster said. "It was a chain reaction."

The winning candidate will likely take office at the beginning of December, Ammons said, and, if he or she does not also win the two-year term, will serve until Jan. 3.

If the same candidate were elected to both terms, he or she would enjoy the benefits of effectively taking office a month early. "They would have one month's seniority and might get better committee assignments and parking spots or whatever," Ammons said.

Assuming the new representative serves for a month, he or she will earn $14,500.

The six candidates who aren't running for the full term — Berry, Sullivan, J. Byron Holcomb and Ruth Morrison, all Democrats; Steven J. Gerdes, a Republican; and Bob Champion, an independent — seem to have kept low profiles. (Sullivan, who was arrested July 26 and charged with drunken driving, is a possible exception.)

None of the six candidates has raised any money, according to the Center for Responsive Politics, and most do not seem to be doing very much campaigning.

"I don't think anybody's doing a whole lot," said Gerdes, 46, who lives in Lynnwood and chose to run thinking he would be the only Republican in the primary.

Berry, 47, the second candidate to file, said he would not have run had he known the race would draw so many candidates. But he hopes the experience will help to position him for a future run for the state Legislature.

Gerdes, meanwhile, seems to view the whole situation with a raised eyebrow.

"I think it's crazy we're electing somebody for 30 days," he said, "and I'm running."

Theodoric Meyer: 206-464-2985 or tmeyer@seattletimes.com. Twitter: @theodoricmeyer.