Ted Haggard, a vocal opponent of gay marriage, resigned as president of the National Association of Evangelicals (NAE) on Thursday after being accused by a male escort of having had a sexual relationship with him and using the drug methamphetamine.
Haggard told a local television channel in his home state of Colorado: "I did call him. I called him to buy some meth. But I threw it away. I was buying it for me but I never used it."
Haggard said that he had sought the man, named as Mike Jones, out at a Denver hotel for a massage but never had sex with him.
Haggard has since temporarily stepped down as senior pastor of the New Life Church in Colorado Springs.
Jones said he went public with his claims on Wednesday to expose what he called the evangelical leader's double standards and denied fabricating his allegations.
"I have said everything that has happened is the truth," he said in a US radio interview.
"I could have made it even worse than it was. But I'm just telling you what happened."
Evangelicals said they were praying for Haggard but were troubled by the situation.
Gary Ledbetter, the director of communications for the Dallas-based Southern Baptists of Texas Convention, said: "This is a blow as the NAE is a significant organisation for us [and] we would always want to lift up a high standard of conduct for church leaders."
The NAE is composed of 60 member denominations representing 45,000 churches across the US.
Conservative Christians are a support base for the Republican Party and George Bush, the US president.
Evangelical leaders have been urging followers to vote in next week's US mid-term elections, with polls showing that Republicans could lose control of one and perhaps both houses of Congress.
They have also encouraged conservative voters in eight states, including Colorado, to support proposed amendments to ban same-sex nuptials.
Matt Foreman, executive director of the National Gay and Lesbian Task Force, said: "They make a career out of defaming gay people and preaching family values, when it's clear that it's just a veneer."