Donald Trump introduced Indiana Governor Mike Pence Saturday as his "first choice" for vice president, citing his host of economic accomplishments as the leader of his central US state.
"I found the leader who will help us deliver a safe society and a prosperous, really prosperous society for all Americans," Trump said.
"He fights for the people, and he's going to fight for you. He is a solid, solid person."
Trump declared he and Pence were the "law-and-order candidates" at his first campaign appearance since naming Pence his running mate a day earlier.
Pence said he had accepted Trump's offer "to run and serve as vice-president of the United States."
"Donald Trump understands frustrations and the hopes of the American people like no leader since Ronald Reagan," Pence said.
Both Trump and Pence sought to draw a contrast between their Republican ticket and presumptive Democratic nominee Hillary Clinton.
"Americans can choose a leader who will fight to make America safe and prosperous again and bring real change, or we can elect someone who literally personifies the failed establishment in Washington, DC," Pence said.
Pence took the former secretary of state to task for a foreign policy that he said had made the US weaker and led to the rise of Islamic State and the recent surge in terrorist attacks in the West.
"America needs to be strong for the world to be safe. On the world stage, Donald Trump will lead from strength," he said.
Word of Trump's likely pick was leaked Thursday, but media reports indicated the billionaire businessman may have sought to change his mind at the last minute in favour of one of his other possible choices, New Jersey Governor Chris Christie or former speaker of the House Newt Gingrich.
Following the Trump-Pence appearance, Clinton's campaign released a statement saying it had been prepared to point out how Pence was "the most extreme pick in a generation -- a doubling down of Trump’s divisive rhetoric and policies."
But, it went on to say, Trump, "after publicly waffling over his own choice," spent more time at Saturday's event attacking Clinton and "talking about his own businesses than his own running mate."
Pence, 57, has served as governor of Indiana since 2013 and also represented the state in Congress for more than a decade.
Trump had signalled that he wanted an experienced lawmaker to help his presidential ticket because he is a political outsider with no experience in governing.
Pence is seen as a fiscal and social conservative who can appeal to a broad swathe of voters in the centre-right party in a bid by Trump to make himself more palatable to some Republicans who have been wary of his candidacy.
Pence's low-key persona stands in contrast to Trump, but Pence will likely do little expand support among critical swing voters with economic and social positions that have little appeal beyond the party's base.
Pence points to his work enacting the largest tax cut in state history and lowering business taxes, balancing the state's budget and efforts to draw businesses to the state.
As governor, Pence came under fire after signing a so-called religious freedom bill that opponents said would have allowed businesses to discriminate against gays, but he later revised the legislation.
Pence has disagreed prominently with Trump on a number of issues, including speaking out against the Republican presidential candidate's controversial call for a ban on Muslims entering the United States and on trade deals.
Pence had been in a tight re-election bid for governor and faced a Friday deadline to withdraw from that race in order to run for vice-president instead.