Kentucky Senator Rand Paul officially launched his campaign in Louisville with a speech that laid bare the difficult challenge he faces in trying to appeal to establishment Republicans while staying true to the Tea Party and libertarian factions that pushed him to national prominence.
Where presidential hopefuls tend to paint in broad strokes during an introductory speech, Dr Paul - an ophthalmologist - notably filled his with some concrete policy positions in an attempt to head off growing criticism of his policies.
For instance, he spent a chunk of his speech outlining his stand on the tentative Iran nuclear deal. While not opposing the deal outright the way some other Republicans have, Dr Paul said he had many concerns.
"It concerns me that the Iranians have a different interpretation of the agreement. They're putting out statements that say completely the opposite of what we're saying. It concerns me that we may attempt, or the president may attempt, to unilaterally and prematurely halt sanctions," he said, adding that he would oppose a deal that does not end Iran's nuclear ambitions.
The remarks came as conservative groups launched a US$1 million (S$1.36 million) ad campaign painting Dr Paul as being weak on Iran.
At the heart of the 52-year-old's balancing act is how he treats the legacy of his father, former congressman Ron Paul, who had himself launched three separate unsuccessful bids for the White House.
The elder Paul was at the announcement and was even acknowledged during the speech but otherwise was a low key presence during the event. Dr Paul has inherited much of his father's passionate young followers by embracing many of the limited government, anti-interventionist principles his father propounded.
His challenge moving forward, it seems, is how to hang on to them while taking up positions that have more appeal to a broader Republican audience.
Foreign policy will likely be the stiffest test as Dr Paul tries to walk away from the isolationist views of his father.
So while he vowed to "do whatever it takes to defend America" from threats like ISIS, he also talked about reducing the flow of US foreign aid.
"Let's quit building bridges in foreign countries and use that money to build some bridges here at home," he said.
Dr Paul added: "We need a national defence robust enough to defend against all attack, modern enough to deter all enemies, and nimble enough to defend our vital interests. But we also need a foreign policy that protects American interests and encourages stability, not chaos."
At home, while Dr Paul could rely on more tried and tested conservative populist messages like reining in the deficit and breaking Washington gridlock, though he also criticised both the Democratic and Republican parties in equal measure.
"Too often when Republicans have won we have squandered our victory by becoming part of the Washington machine. That's not who I am," he said.
From Kentucky, Dr Paul is due to embark on a tour of four states that have early party elections to pick nominees. His declaration - the second one from a Republican - is expected to be followed by a slew of others. Florida Senator Marco Rubio is due to announce his candidacy next week.
Latest polls show Dr Paul lagging behind establishment-backed party contenders like Wisconsin governor Scott Walker and former Florida governor Jeb Bush.
