Of all the places to hear fulminations against President Obama, one of the least expected is the corner of 71st Avenue and Queens Boulevard, in the heart of a Congressional district that propelled Democrats like Geraldine A. Ferraro, Charles E. Schumer and Anthony D. Weiner to Washington.

But it was there that Dale Weiss, a 64-year-old Democrat, approached the Republican running for Congress in a special election and, without provocation, blasted the president for failing to tame runaway federal spending. “We need to cut Medicaid,” she declared, “but he won’t do that.” She shook her head in disgust. “He is a moron.”

After nodding approvingly for a time, the Republican candidate, Bob Turner, signaled for an assistant to cut off Ms. Weiss. Frustration with Mr. Obama is so widespread, he explained later, that he tries to limit such rants to about 30 seconds, or else they will consume most of his day.

“It’s endemic in the district,” Mr. Turner said. “You can’t stop them once they get started.”

The Sept. 13 election for the Ninth Congressional District seat became vacant this summer when Mr. Weiner quit. The race was widely viewed as a sleepy sideshow – a mere formality that would put David I. Weprin, a Democratic state assemblyman and heir to a Queens political dynasty, into a seat known for its deep blue hue.

Instead, the race has become something far more unsettling to Democrats: a referendum on the president and his party that is highlighting the surprisingly raw emotions of the electorate.

National Democrats, alarmed by a poll that showed the contest far closer than anticipated, are privately fretting that even a close outcome in a working-class swath of Brooklyn and Queens may foreshadow broader troubles for the party in 2012.

The Siena College poll, conducted early this month, showed Mr. Weprin with an advantage of 6 percentage points, within the margin of sampling error of plus or minus 4 points.

Suddenly Mr. Weprin’s aides have ramped up fund-raising, enlisting big-name figures like Senator Joseph I. Lieberman to headline events. The Democratic Congressional Campaign Committee has dispatched operatives to advise the candidate. And the campaign, aided by big city unions, is drawing up an extensive field operation to turn out the vote.

Few predict a Republican upset: registered Democrats outnumber Republicans by three to one in the district. But it is not uniformly liberal – many Orthodox Jews live there, for example – and even those closest to Mr. Weprin grudgingly describe the contest as uncomfortably competitive.

On paper, Mr. Weprin seems like a sturdy candidate; he is the former chairman of the City Council’s Finance Committee and the son of an Assembly speaker. His message seems tailor-made for the district: he promises to protect Medicare and raise taxes only on the super-rich.

But the election, waged with little news media attention, offers scant time to remind voters of his biography. And after a long summer of stock market gyrations and battles over the federal debt, voters seem determined to register their frustrations with Washington.

“The issue defining this race,” said Robert Zimmerman, a member of the Democratic National Committee from New York, “is the confidence that the electorate has in this district about the national Democratic agenda.”

In much the way that Mr. Turner tries to link Mr. Weprin with the White House, Mr. Weprin seems determined to identify Mr. Turner with the Tea Party. He refers to Mr. Turner, 70, as “my Republican, Tea Party conservative opponent,” even though Mr. Turner avoids the Tea Party label.

In an unusual tactic, Mr. Turner, a Roman Catholic, is directly appealing to Jewish voters by criticizing Mr. Obama’s policies on Israel, despite the fact that Mr. Weprin is an Orthodox Jew.

But in the end, the race is likely to turn on the district’s die-hard Democrats. Special elections generally draw little attention or enthusiasm: advisers to both candidates predict that in a district with more than 300,000 registered voters, as few as 50,000 will vote.

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