He also had to humanize Romney, one of the main efforts of the evening. By the looks of the audience, he did it.
Rubio focused on family, both his own and Romney's, and the American dream of immigrating to the United States and becoming successful here, or helping your children to do so.
"They struggled through poverty and the great depression," he said of Romney's father's family. "And yet he rose to be an admired businessman, and public servant. And in November, his son, Mitt Romney, will be elected president of the United States."
Rubio is often used by Republicans as a surrogate to the Latino community, andaddressed Latino voters in a video earlier in the evening about why they should vote for GOP candidates. The message in that video was, in part, that the party's economic plans would better serve Latinos, who have higher unemployment than the nation's average, but mostly prefer Democrats.
He didn't address Latinos specifically in his speech, although he talked about his grandfather's emigration from Cuba and threw in a line in Spanish. His focus, though, was largely on the economy, and specifically aimed at people who are struggling.
"Yes, we live in a troubled time," Rubio said. "But the story of those who came before us reminds us that America has always been about new beginnings. And Mitt Romney is running for president because he knows that if we are willing to do for our children what our parents did for us, life in America can be better than it has ever been."
Some of the biggest crowd reactions came when he wasn't talking about Romney. One of the best-received portions was an anecdote about his father, who worked for years at a bar.
"He stood behind a bar in the back of the room all those years, so one day I could stand behind a podium in the front of a room," Rubio said, bringing in a huge applause.