Sen. Joseph I. Lieberman of Connecticut is challenging the Internal Revenue Service’s investigation into alleged political activities of the United Church of Christ and is asking the IRS to explain why it began the inquiry.

Mr. Lieberman, a Connecticut independent, said he found the investigation “especially troubling because of the service’s inadequate guidance” for churches trying to follow federal law when they invite candidates to appear at gatherings.

The IRS notified the United Church of Christ in February that the government had a “reasonable belief” that the church had engaged in forbidden political activities because Sen. Barack Obama spoke at a major conference held by the church last year in Hartford. The IRS also said that 40 volunteers for Mr. Obama, an Illinois Democrat, “staffed campaign tables outside the [convention] center to promote” Mr. Obama’s presidential efforts.

Under federal law, churches and charities must not participate in a political campaign in support of, or in opposition to, a candidate for public office. An organization that violates the law a single time could be forced to pay a penalty fee and have its tax exemption revoked.

Officials of the United Church of Christ denied any wrongdoing.

Senator Lieberman told the IRS that the church “took significant precautions to ensure that Senator Obama’s appearance” satisfied legal requirements.

“Throughout my career in the Senate, I have supported the strong and fair enforcement of our nation’s tax laws, including laws applicable to religious institutions,” wrote Mr. Lieberman.

“But I am concerned about the chilling effect on legitimate activity by religious organizations that results from initiating a church tax inquiry without first satisfying the reasonableness standard, and I am further concerned by the lack of clear guidance in this area,” Senator Lieberman said.

Mr. Lieberman, the Democratic nominee for vice president in 2000, this year has endorsed Sen. John McCain, the Arizona Republican, for president.