Ron Paul, father of Rand Paul, has always been an original thinker.

As a longtime member of Congress from Texas, on paper a Republican but in posture a Libertarian, Congressman Paul rarely met a spending bill he could like. He was an avowed isolationist and civil libertarian, voting against the Iraq War and the USA Patriot Act. His nickname on Capitol Hill was “Dr. No.”

Paul was, and still is, the hero of the “liberty” movement. His three presidential campaigns – once as the Libertarian Party nominee and twice as a Republican – were a testament to his devotion to the cause. And his voters were devoted to him like no other candidate’s supporters.

Now his son, Senator Paul (R) of Kentucky, is following in his footsteps – sort of. He’s more “libertarian-leaning” than big-L Libertarian. He’s a physician like his dad, but no one confuses him with Dr. No.

When Paul launches his presidential campaign Tuesday at the Galt House Hotel in Louisville, Ky., his father will be there. But his presence will reflect an uneasy reality for the freshman senator: The junior Paul needs his dad, and a similar devotion of core supporters, even as he needs to create some distance from him.

“His father had a basic floor of support below which he wasn’t going to go, but he also had a ceiling,” says Dennis Goldford, a political scientist at Drake University in Des Moines, Iowa.

“The son seems to have a floor – those so-called liberty and tea party Republicans – but does he have a ceiling? He’s been trying to expand that, and reach out to younger people and minorities. We don’t know how that might work.”

Paul is not the only political scion competing for the 2016 Republican presidential nomination. Jeb Bush, son and brother of former presidents, is expected to jump in soon. For both Paul and Mr. Bush, the family name is a help and a hindrance. “I am my own man” is a core message.

But their challenges are different. The former Florida governor is grappling with “Bush fatigue,” precisely because his family has been successful at the highest levels of politics (three successful presidential campaigns) and because of the failings of his brother’s presidency, namely an unpopular war and a near economic collapse. “Paul fatigue” isn’t an issue. Rather, the younger Paul has to distance himself from his father precisely because he has views that are firmly outside the mainstream – views that made his brand lively and unique, but also made it impossible for him to be elected president.

And unlike the former Presidents Bush, Ron Paul is still a public activist, adding to his son’s challenge.

A Ron Paul tweet Monday illustrates the point.

Join me at University of Texas this Saturday for a free conference on ending the phony wars on drugs and terrorism: http://t.co/lm7sjwUC8s — Ron Paul (@RonPaul) April 6, 2015

Adam Brandon, executive vice president of the libertarian group Freedomworks, recalls a TV interview with Ron Paul during his 2012 presidential campaign, in which he was asked what he would do on Day 1 as president. Paul said he hadn’t thought about it.

“That was because Paul was running to advance the liberty movement,” says Mr. Brandon. “He wasn’t running necessarily to be president.”

The senior Paul also wasn’t looking to be vice president, the consolation prize that most candidates keep in the back of their minds, he notes.

Rand Paul is playing a more mainstream game, certainly with an eye on the top prize. Paul has had his moments in the spotlight as a maverick. He conducted a true “talking filibuster” in 2013, speaking in opposition to John Brennan’s nomination as Central Intelligence Agency director. Paul objected to the Obama administration’s use of drones.

"I will speak as long as it takes,” Paul said, “until the alarm is sounded from coast to coast that our Constitution is important, that your rights to trial by jury are precious, that no American should be killed by a drone on American soil without first being charged with a crime, without first being found to be guilty by a court."

Paul has also kept up his father’s focus on the Federal Reserve, though for the younger Paul, the goal is to audit the Fed, not to end it, as his father wants.

The Rand Paul bandwagon is certainly organized. Three years in a row, including this past February, Paul has won the presidential straw poll at the Conservative Political Action Conference, an echo of his father’s victories in the same straw poll just four and five years ago.

But whether Rand Paul can build significant support beyond that core constituency – at CPAC, dominated by younger voters – is the big question as he launches his 2016 campaign.

The early nominating contests hold some promise: Iowa and Nevada both hold caucuses, a format that favors candidates with intensely committed supporters, which Paul has. New Hampshire often goes “establishment” in its first-in-the-nation primary, but the “Live Free or Die” state also has a strong libertarian streak, which could benefit Paul.

Paul’s father came in second in the New Hampshire primary in 2012 with 23 percent of the vote, to Mitt Romney’s 40 percent. But he wasn’t able to build on that in subsequent states. Rand Paul is well known and reasonably well-liked in New Hampshire, but the expected 2016 GOP field is stronger than that of 2012. Paul might be lucky to match his father’s 23 percent, and even then, would he be able to build on that?

“The question is what [Rand Paul’s] ‘crossover’ will look like – meaning, can he find a way to cross over and emerge from his father’s niche and appeal to a more mainstream Republican in New Hampshire,” says Dante Scala, a political scientist at the University of New Hampshire, Durham.

A big challenge for Paul in selling himself to Republican voters is his noninterventionist approach on foreign policy. The 2016 race could end up being dominated by security concerns, given the rise of the Islamic State and the Iran nuclear challenge. By the morning of his announcement day, Paul had yet to react publicly to the Obama administration’s nuclear framework accord with Iran. But one group didn’t hesitate to use an old Paul statement on Iran to attack him.

“You know, it’s ridiculous to think that they’re a threat to our national security,” Paul said of Iran in 2007, defending his father’s views.

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The ad’s sponsor, the Foundation for a Secure and Prosperous America, is spending $1 million to run it in the four early primary states, according to The New York Times.

“Obviously, national security and foreign policy are going to be tricky for Rand Paul,” says Republican strategist Ford O’Connell. “If they are high on voter and donor interest, he’ll have a hard time gaining traction.”