Pete Buttigieg has officially launched his bid to become the Democratic Party nominee for the 2020 presidential election and challenge Donald Trump for the White House.
He held his launch event in South Bend, the Indiana town where he has served as mayor since 2012.
Setting out his vision to "re-imagine our future", he promised a "totally different" politics from Mr Trump.
Should he win, Mr Buttigieg would be the first openly gay US president.
At 37, he is the youngest entrant in a crowded field of Democratic candidates that features Vermont Senator Bernie Sanders, 77, and 69-year-old Massachusetts Senator Elizabeth Warren.
Despite his relative inexperience, the former Rhodes Scholar has surged in the polls in recent weeks, propelling him from the margins to the front of the pack.
In a veiled attack on the Trump administration, he railed against the "myth being sold" by those who sought to "stop the clock and turn it back".
"It is time to walk away from the politics of the past and toward something totally different," he told supporters at his launch event.
His campaign has so far focused on gun control, climate change and LGBT rights and other progressive issues he hopes will appeal to millennial voters.
Mr Buttigieg's track record as mayor of South Bend, which has seen a struggling economy improve during his tenure, has also earned him plaudits.
Since announcing his presidential run in January, Mr Buttigieg has garnered a wave of attention from the media and voters, raising more than $7m (£5.3m) in funds in the first quarter of 2019.
Analysts believe he could appeal to both moderates and progressives in the Democratic Party.
Opinion polls in Iowa and New Hampshire last week put Mr Buttigieg third in both states, behind Bernie Sanders and former Vice-President Joe Biden.
Mr Sanders launched his run last month, while Mr Biden, who has faced allegations of inappropriate touching by several women, is yet to announce his candidacy.
At the rally, Mr Buttigieg sought to differentiate himself from rivals as a candidate taking the "long view", especially on issues such as climate change.
"I recognise the audacity of doing this as a Midwestern millennial mayor," he said.
"More than a little bold… But we live in a moment that compels us each to act. The forces changing our country today are tectonic."
More than a dozen rivals are running to be the Democratic Party candidate for the presidential election in November 2020.
The process of selecting a nominee will begin early next year.
Who is Pete Buttigieg?
Born in South Bend in 1982, Mr Buttigieg studied at Harvard and Oxford, graduating from the latter in 2007 with first-class honours in philosophy, politics and economics.
He went on to work as a consultant at McKinsey & Company between 2007 and 2010 and served for seven months in Afghanistan as a Navy reservist in 2014.
At 29, he was elected mayor of South Bend, taking office in January 2012.
Despite an unsuccessful run for chairman of the Democratic National Committee in 2017, former President Barack Obama name-checked him as a rising star.
The video announcing he will run for president focuses on his role in the Rust Belt city's redevelopment and also takes aim at corruption and infighting in Washington.
He came out as gay in 2015 and married Chasten Glezman, a middle school teacher, last year.
What does he stand for?
In his rally speech, Mr Buttigieg positioned himself as the voice of millennials. Critics say his campaign has so far emphasised values and his personal story rather than a distinctive policy agenda.
He has, however, expressed support for:
- Tougher action on climate change, including the so-called "Green New Deal" proposals
- Gun control reform, namely universal background checks
- A wealth tax to fund government spending
- The withdrawal of US troops from Afghanistan
- Passing the Federal Equality Act, legislation that would give federal non-discrimination protections to LGBT people
Who will take on Trump in 2020?
Pete Buttigieg has joined the race to stop Donald Trump from being re-elected. But who else has a shot at becoming the next president?