The 2012 Republican nominee had a distrustful relationship with some conservatives during his presidential run, but he was warmly received by hundreds of activists at the Conservative Political Action Conference, the annual meeting of activists from that wing of the party.
Romney's call echoed his 2012 campaign promise to expand the Pentagon budget.
Romney advised anyone who would be President to "do whatever you can to keep America strong, to keep America prosperous and free and the most powerful nation on earth." He added that "American's pre-eminent position is not guaranteed," mentioning China, Russia and "the jihadists" as forces that could surpass the United States this century.
Although he singled out his former running mate, Paul Ryan, for praise, Romney did not address the spending plan that the House Budget chairman released this week. It would devote $2 trillion less for defense over the next 10 years than Romney and Ryan proposed in the campaign.
Greeted by loud applause and cheers of "Mitt, Mitt," the former Massachusetts governor, who has said he is done with elective politics after two unsuccessful presidential runs, began by thanking the conservative crowd for its past support.
"Of course, I left the race disappointed that I didn't win," he said, adding that setbacks in the past had paved the way for future Republican victories. "It's up to us to make sure that we learn from our mistakes, and my mistakes."
Romney said he rejected the "fashionable" pessimism about the future of the conservative movement and the Republican Party.
"Like you, I believe a conservative vision can attract a majority of Americans," he said, acknowledging that, "as someone who just lost the last election," he is probably not the best source of advice on a way forward.
His 15-minute speech was largely thematic and sidestepped divisions within the GOP over efforts to broaden the party's appeal. In a nod to the isolationist sentiment that runs through much of the young, libertarian crowd, he leavened his praise for U.S. military interventions in Iraq and elsewhere by injecting the phrase "whatever you think of them."
Republicans would do well to look to its state governors for solutions to the nation's problems, Romney said. He singled out several, including Georgia's Nathan Deal, for promoting charter schools, and Michigan's Rick Snyder, who signed legislation designed to weaken the power of labor unions.
Romney has often sought out CPAC as a venue for important appearances, even though many of those attending the annual events remained suspicious of the depth of his conservatism.
In 2008, he pulled out of the Republican primary campaign at a CPAC conference. Four years later, in another CPAC speech, he declared himself "severely conservative," a description that struck some as a ham-handed appeal to the ranks.
This time, he appeared to flub a date likely to go down as the most pivotal of his life, his defeat in the Nov. 6. election. "We may not have carried on Nov. 7, but we haven't lost the country we love and we haven't lost our way," Romney said.