Macklemore & Ryan Lewis are one of the hottest acts on pop radio these days, having scored No. 1 hits with the lighthearted novelty song “Thrift Shop” and its bouncy follow-up “Can’t Hold Us.”

That is no small feat for a rap duo from Seattle without a major-label contract, but their single “Same Love” has the potential to be groundbreaking in a different way: it is the first song to explicitly embrace and promote gay marriage that has made it into the Top 40, gay-rights advocates and radio executives said.

“The fact that a song solely dedicated to the message of marriage equality is climbing the charts and quickly becoming a popular song across the country is a big deal,” said Charlie Joughin, a spokesman for the Human Rights Campaign, the largest gay rights organization. “It’s indicative of a changing attitude.”

Radio programmers say “Same Love” seems to have captured the nation’s mood, reflecting growing support among young adults for gay marriage in polling. It also hit the charts in February, just as several states began taking up the issue and the Supreme Court was considering challenges to the federal Defense of Marriage Act and to California’s ban on same-sex marriage.