Rubio took aim at several of the issues that have become mainstays of Trump's pitch, including support for the military, pushing back on China's moves in the international community and providing for veterans.
He called for a "Reagan-style rebuilding of the military," noted China's military moves and stressed issues at the Veterans Affairs, saying of those contributing major delays, "you will be fired from your job."
Shortly before Rubio went on stage, Trump sought to brandish his own ability to turn out voters in a pitch to sell himself as a viable general-election candidate for Republicans, pointing to his support in states this month.
"I am growing the Republican Party tremendously — just look at the numbers, way up! Democrats numbers are significantly down from years past," Trump tweeted.
Rubio pushed back while leaning on his upbringing to parents who worked in the hotel industry, saying again, "We will not just unite it, we will grow this party."
"When I'm our nominee, all of these folks that work in the hotel rooms here, that clean the hotel rooms like my mom did, that serve the drinks behind the bar like my father did, all of these hotel employees — they haven't heard from us in a long time," Rubio said. "When I'm our nominee they will hear from us because I come from where they live."
Establishment Republicans have urged supporters to line up behind Rubio, Cruz or Ohio Gov. John Kasich as an alternative to Trump, who has picked up wins in New Hampshire and South Carolina and could win in Nevada on Tuesday.
"This election can't just be about making a point. It can't just be about electing the loudest person in the room," Rubio said. "It has to be about electing someone first and foremost that will win."
"I will win, and the Democrats know I will win," Rubio said. "I'm a conservative that can unite this party."
"I will never divide you against each other to win an election, I will never seek to make you angry at another group of Americans so that you vote for me instead," Rubio said during an extended riff aimed at Trump, without naming the front-runner.
"You also have to have a president that loves all of the American people, even the ones that don't love you back," Rubio said, mentioning getting protesters at his rallies and people saying "nasty" things about him on Twitter, adding to laughs, "You know what? I'm going to cut their taxes too."
Rubio is heading Tuesday afternoon to Michigan and Minnesota, which hold voting early next month.