And Bobby Jindal, the Louisiana governor, said the most important moment in his life was not his wedding day or when he held his first child, but “the moment I found Jesus Christ.”

Although the percentage of Iowans who are evangelical Christians is no greater than the national average — about one in four, according to the Pew Research Center — they represent nearly 60 percent of the Republican caucus turnout. Their outsize influence in Iowa and other primary states has pulled the Republican field to the right in the past, especially on issues like abortion and immigration, and led critics to discount the caucus as harmful to the party’s nominee in the general election.

A number of the candidates were adamant about opposing same-sex marriage despite the view of some national Republican strategists that the party is losing touch with younger voters on the issue.

Mr. Rubio, a Florida senator, defended traditional marriage. “I remind people that the institution of marriage as one man and one woman existed long before our laws existed,” he said. “Thousands of years of human history teach us a simple truth: The ideal way to raise children is when a mother and father married to each other, living in the same house, raise children together.”

At a house party nearby earlier in the day, Mr. Rubio, making his first visit here as a declared candidate, indicated he would compete aggressively in Iowa. “We want to win the caucuses in this state,” he said.