House Minority Leader Nancy Pelosi said Monday that the IRS should be condemned for targeting conservative groups for special scrutiny in the run-up to last year’s elections, but she also blamed the Supreme Court for opening the door to broader political activity.

In the Citizens United decision, the court ruled 5-4 that corporations have First Amendment political rights and ruled that while they cannot contribute directly to candidates, they can run ads making their own views known.

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Mrs. Pelosi, the top Democrat in the House, said that has become a problem for the IRS in enforcing the laws.

“There needs to be more clarity in the law regarding the activities of tax-exempt organizations along with greater disclosure and transparency. We must overturn Citizens United, which has exacerbated the challenges posed by some of these so-called ‘social welfare’ organizations,” she said.

She also called for “appropriate action, without any delay or hesitation” to make sure the IRS is playing fairly.

David N. Bossie, president of Citizens United, the group that brought the initial lawsuit that the Supreme Court ruled on, said Monday Mrs. Pelosi should go after the IRS, not conservative groups.

“It appears that the left will stop at nothing in its quest to silence those who believe differently than them,” Mr. Bossie said in a statement, adding that Mrs. Pelosi’s attack was misguided and that “nowhere in Pelosi’s statement released today does she call for a congressional investigation.”

Democrats have tried for three years to overturn the Citizens United decision, including President Obama, who used his 2010 State of the Union speech to confront Chief Justice John G. Roberts and his colleagues, sitting in the front of the House chamber.

But their efforts have repeatedly fallen short, and in last year’s election Democrats, including Mr. Obama’s supporters, used the same organizations to aid his re-election bid.

Democrats said it made no sense to disarm themselves if conservatives were making use of the new tools.