As a political talent, the newest official entry in the Republican race, Marco Rubio, has a lot going for him. The son of a maid and an immigrant bartender from Cuba, Mr. Rubio is, at 43, young, handsome, and one of the best speakers I’ve seen.

He is good — really good — at articulating two ideas: first, that America is and should be a land of upward mobility and opportunity and economic growth, and second, that America has a responsibility to promote and defend freedom around the world. The power of those ideas helped propel both John F. Kennedy and Ronald Reagan to the presidency.

But the Rubio candidacy is not without formidable risks, as well, and at the risk of raining on opening day, I’m going to dwell on some of them in this column, on the theory that it’s better to know about these risks up front, when someone is running for president, than to discover them only after an election.

The first risk might be called biography risk. To be blunt about it, a one-term senator in his early 40s with a law degree, young children, a golden tongue, and an inspiring personal story sounds a bit familiar — too familiar, perhaps, to many of us who have lived through the Obama years.

Like Mr. Obama, Mr. Rubio would be a “first” — first Hispanic president. Mr. Rubio runs the risk of making it all about his own personal story rather than about the country and what it needs. Though Mr. Rubio, unlike Mr. Obama, did serve two years as speaker of the Florida House of Representatives, he doesn’t have a lot of executive experience.

If your ideal presidential candidate is a kind of David Broder ideal of a governor with an impressive track record that has been ratified by voters with re-election, Mr. Rubio doesn’t fit the bill.

Even as a senator, Mr. Rubio’s track record has been less than impressive.

His signature legislative initiative was immigration reform. In 2013 Mr. Rubio joined seven other senators — Messrs. McCain, Flake, and Graham on the Republican side, and Messrs. Schumer, Durbin, Menendez, and Bennet on the Democratic side — to introduce a Border Security, Economic Opportunity, and Immigration Modernization Act.

Under pressure from anti-immigration Republicans, he folded faster than a cheap pup tent. No immigration reform has passed, and close observers of the senator and the issue in Washington were left wondering how a President Rubio would stand up to ISIS or Ayatollah Khamenei, let alone Congress, if he couldn’t even handle some heat from Rush Limbaugh and his listeners.

The best face to put on this would be that Mr. Rubio listened and rethought rather than trying to ram through a bill with elite support over popular opposition. But leadership from the senator was absent, and partly as a result, a lot of people who, like Mr. Rubio’s father, want to pursue their dreams in America and could contribute here are stuck, locked outside the border by ridiculous immigration laws.

Finally, one could make the case that the problem with Mr. Obama was less his biography than his policies. But most importantly, Mr. Rubio’s policy so far on the tax issue has been as much of a disappointment as his immigration flip-flop.

As the Coolidge Foundation’s Amity Shlaes and Mathew Denhart have pointed out in a devastating series of articles for National Review and the Wall Street Journal, Mr. Rubio’s tax plan would leave the top marginal individual income tax rate at 35% — higher than the 28% at the end of the Reagan administration or the 31% at the end of the George H.W. Bush administration.

The Rubio plan would also lower the bracket at which the top rate applies, to $75,000 for singles or $150,000 for couples. When voters realize this, they may ask, “For this, we need Republicans?” And they may conclude, “no, thanks.”

Maybe the dynamics of the Republican primary field will emerge so that Mr. Rubio gets traction. Maybe he will make his mark four or eight years from now. Or maybe he will make his mark this time around as a vice presidential candidate. He has a lot to contribute. But anyone tempted by his formidable political gifts would be wise also to keep in mind the risks.

Mr. Stoll is editor of FutureOfCapitalism.com.