WASHINGTON — For months during last year’s presidential race, conservatives sought to tar Barack Obama’s campaign with allegations of voter fraud and other transgressions by the national community organizing group ACORN, which had done some work for the campaign.

But it took amateur actors, posing as a prostitute and pimp and recorded on hidden cameras in visits to ACORN offices, to send government officials scrambling in recent days to sever ties with the organization.

Conservative activists and broadcasters were gleeful about the success of the tactics in exposing ACORN workers, who appeared to blithely encourage prostitution and tax evasion. It was, in effect, the latest scalp claimed by those on the right who hope to weaken the Obama administration by attacking its allies and appointees they view as leftist.

The ACORN flap came a week after the resignation of Van Jones, a White House environmental official attacked by conservatives, led by Glenn Beck of Fox News Channel, for once signing a petition suggesting that Bush administration officials might have deliberately permitted the Sept. 11, 2001, terrorist attacks.

Even before Jones stepped down, Beck had sent a message to supporters on Twitter urging them to “find everything you can” on three other Obama appointees.

Conservatives believe they have hit upon a winning formula for such attacks: mobilizing people to dig up dirt, trumpeting it on talk radio and television, prompting Congress to weigh in and demanding action from the Obama administration.

In response to the ACORN videos, which became an instant hit on YouTube, the Senate voted 83-7 on Monday to prohibit the Department of Housing and Urban Development from giving federal housing money to the organization. The bill’s advocates said the group had received $53 million in such financing since 1994.

Last Friday, the Census Bureau dropped ACORN as one of 80,000 national unpaid “partners” helping promote the 2010 census, saying the recent scandals involving ACORN affiliates meant that the group’s involvement might “create a negative connotation” and discourage participation in the population count.

On Tuesday, the House Republican leader, Rep. John A. Boehner of Ohio, wrote to Obama asking him to cut off all federal financing to ACORN and its affiliates. “It is evident that ACORN is incapable of using federal funds in a manner that is consistent with the law,” Boehner wrote.

The undercover videos showed a scantily dressed young woman, Hannah Giles, posing as a prostitute, while a young man, James O’Keefe, played her pimp. They visited ACORN offices in Baltimore, Washington, Brooklyn and San Bernardino, Calif., candidly describing their illicit business and asking the advice of ACORN workers. Among other questions, they asked about how to buy a house to use as a brothel employing under-age girls from El Salvador.

O’Keefe, 25, a filmmaker and conservative activist, was dressed so outlandishly that he might have been playing in a risque high school play. But in the footage made public — initially by a new Web site, BigGovernment.com — ACORN employees raised no objections to the criminal plans. Instead, they eagerly counseled the couple on how to hide their activities from the authorities, avoid taxes and make the brothel scheme work.

On one of the videos, an unidentified ACORN employee in Washington, told that the pair were engaging in prostitution, explained how to disguise their activities in dealing with bankers and the government.

“You don’t put down ‘I’m a prostitute’ or ‘I’m a lady of the night and this is where I’m getting my income,”‘ the ACORN worker said.

At the Baltimore office, a helpful worker suggested describing the prostitute’s work on a loan application as “freelance performing artist” and explaining that she and the pimp might want to claim some of the young Salvadoran prostitutes as dependents and collect the child tax credit for them.

In an account of their escapades at BigGovernment.com, O’Keefe explained how he and Giles, a 20-year-old college student, chose their methods against ACORN. “Instead of railing against their radicalism, it is best to bring out this type of radicalism,” he wrote. They decided upon “posing the most ridiculous criminal scenario we could think of and seeing if they would comply —which they did without hesitation.”

In a weekend statement, Bertha Lewis, the chief organizer for ACORN, asserted that the bogus prostitute and pimp had spent months visiting numerous ACORN offices, including those in San Diego, Los Angeles, Miami and Philadelphia, before getting the responses they were looking for.

“I cannot and I will not defend the actions of the workers depicted in the video, who have since been terminated,” Lewis wrote. But she defended the group’s overall record and said it had become “the boogeyman for the right-wing and its echo chambers.”

Robert L. Borosage, co-director of the liberal Campaign for America’s Future, called the tactics used to target Jones and ACORN “McCarthyite,” harping on minor failings and distorting admirable overall records. “This is dangerous stuff,” he said. “I don’t think progressives will sit back and let this gain momentum.”

But Mike Gonzalez, vice president for communications at the conservative Heritage Foundation, said the episodes simply reflected a Web-based democratization of investigative reporting, made necessary in part by the failures of the mainstream media, including The New York Times. “It should have been ‘60 Minutes’ doing this stuff — not two people whose combined ages are 45,” he said.

ACORN describes itself as the nation’s largest grass-roots community organizing group, claiming 400,000 low- and moderate-income families in 110 cities as members. Founded in Arkansas in 1970, it has worked in recent years for higher minimum wages, more affordable housing and voter registration, among other causes.

It was ACORN’s election activities that drew opponents’ attention last year, including registration cards filled out by ACORN workers in the name of Mickey Mouse and other imaginary voters. Republicans highlighted the fact that the Obama campaign had paid more than $800,000 to an ACORN affiliate for get-out-the-vote efforts. ACORN officials replied that they were the ones who had uncovered and reported most cases of fraud by a small number of the organization’s workers.