WASHINGTON — Sen. Joe Lieberman of Connecticut is questioning the Internal Revenue Service’s investigation of the United Church of Christ for a speech Sen. Barack Obama gave at the denomination’s national meeting last year after he began his presidential run.

Lieberman asked the IRS in a letter to explain how it could meet the required threshold of a “reasonable belief” that a church has engaged in improper political activity if had not contacted church officials before starting the investigation.

He also called the inquiry “especially troubling” because the agency hasn’t given churches adequate guidance about what’s allowed.

“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 Lieberman, an independent and former Democratic vice presidential candidate. “But I am concerned about the chilling effect on legitimate activity by religious organizations ...”

Obama, a member of Trinity United Church of Christ in Chicago, spoke about faith and public life at the denomination’s General Synod in Hartford, Conn., last June.

The IRS said in the letter, made public by the denomination, that it was concerned about articles posted on the church’s Web site and other sites stating Obama had addressed nearly 10,000 people at the event. The agency also said Obama volunteers had staffed campaign tables “outside the center to promote his campaign.”

The Rev. J. Bennett Guess, a spokesman for the Cleveland-based denomination, said a group of Obama volunteers was outside the Hartford Civic Center, where the event took place, but that they were told they could not enter the meeting.

Church leaders consulted with lawyers before the event on following IRS rules, Guess said. Before Obama spoke, a top church official told the crowd that the senator’s talk was not a campaign-related event and that no leaflets or other signs of political support would be allowed.

The IRS does not comment on investigations.