Florida Senator Marco Rubio has urged Americans to look to the future as he launched his 2016 White House campaign.
Mr Rubio, 43, said in a Miami speech he would "lead the way toward a new American century".
He presented the presidential election as "a generational choice" and warned against going back to "the leaders and ideas of the past".
He is the third Republican to officially announce a candidacy after Senators Rand Paul and Ted Cruz.
It comes a day after former Secretary of State Hillary Clinton said she would stand for the Democratic nomination.
Referring to her but not by name, Mr Rubio said: "A leader from yesterday began a campaign for president by promising to take us back to yesterday."
Mr Rubio, a Cuban-American, has been a harsh critic of President Barack Obama's policies, especially on immigration and the diplomatic thaw with Cuba.
He said in his speech that his poor Cuban parents had lived the American Dream but he questioned whether that dream was still possible due to the burden of taxes and regulation.
The Florida senator, who was first elected in 2010, holds conservative positions on government and military spending, abortion and negotiations with Iran over its nuclear programme.
He was previously criticised by some Republicans for initially supporting a bipartisan Senate immigration reform bill.
He has since said that border security must be strengthened before any change, criticising Mr Obama's executive actions on immigration.
Analysis - Anthony Zurcher, BBC News, Miami
Privileged opponents aren't the only obstacle on Mr Rubio's path to the presidency.
Many Republican activists view him as a Tea Party darling-turned-heretic, thanks to his sponsorship of ill-fated immigration reform legislation that included a path to permanent residency for millions of undocumented workers.
It was Mr Rubio's opportunity to have a signature achievement - and ended up being a singular disaster, as the party's rank-and-file revolted.
He eventually renounced his own plan, and immigration reform merited barely a mention in Monday night's speech.
Instead there were the obligatory red meat references to foreign policy - "when America fails to lead, global chaos inevitably follows" - and talk of single mothers, students and janitors struggling to make ends meet.
Mr Rubio would be the first Hispanic president if he won, as would Texas Senator Ted Cruz.
The field for the Republican nomination is likely to be crowded, with former Florida Governor Jeb Bush and Wisconsin Governor Scott Walker also expected to run.
Mrs Clinton, the first major candidate on the Democratic side to declare, is travelling to Iowa and other states, seeking to meet voters before a more formal rally in May.
2016 runners and riders
- An early (undeclared) Republican frontrunner is Jeb Bush
- Hillary Clinton will have learnt much from her failed campaign of 2008
- New Jersey Governor Chris Christie could battle Bush for the party's centre ground
- Texas Senator Ted Cruz is a darling of the Tea Party
- Libertarian Rand Paul has his supporters - and enemies - among Republicans
- Maryland Governor Martin O'Malley has already begun an undeclared Democratic campaign