Sen. Ted Cruz (R-Texas) was first out of the gate in the 2016 presidential horse race, and Sen. Marco Rubio (R-Fla.) is the underdog everyone suddenly seems to be rooting for. Former Florida Gov. Jeb Bush (R) is still probably the odds-on favorite to win the Republican presidential nomination, with Wisconsin Gov. Scott Walker not too far behind.

But this is the only prediction I'm comfortable making: Sen. Rand Paul (R-Ky.), who is announcing his candidacy in Louisville today, won't be America's next president.

That's not because the 52-year-old freshman senator from Kentucky wouldn't make a formidable candidate; it's because he won't get the chance. Republicans won't nominate him as their 2016 standard-bearer — luckily for Democrats.

Paul isn't necessarily a natural candidate — he can be a little unflatteringly churlish with the news media, a candidate's gateway to voters — but he doesn't come across as phony, and campaigning is a skill candidates can and do acquire on the trail. His campaign theme will be the aggressively populist couplet "Defeat the Washington machine. Unleash the American Dream":

Paul's teaser video gets a little messianic near the end, but the quotes he includes from various pundits seem pretty accurate: He is the most interesting candidate in the race, he is a "different kind of Republican," he does take some unpopular and principled stands, and he would appeal to more young and minority voters than the typical Republican candidate.

In his pre-announcement buildup, Paul touts himself as someone who "can beat Hillary Clinton" and "will broaden the reach of the Republican Party." That second part may be true. Paul ventures out of the GOP comfort zone to address black audiences, hipsters, the technorati, and other demographics not traditionally associated with the Republican Party, and he addresses substantive issues like prison sentencing reform, voting rights for felons, and curbing drone warfare that won't win him any Tea Party straw polls.

Perhaps most appealingly, Paul is a relative dove in a party that is increasingly returning to its hawkish posture after a brief flirtation with military restraint. On Sunday, Sen. Lindsey Graham (R-S.C.) dropped a superfluous dig at Paul in a critique of the Obama administration's framework nuclear deal with Iran: "I think everybody on our side except maybe Rand Paul could do better." But here's the thing: A sizable majority of Americans support an Iran deal, as The Week's William Falk points out.

Paul isn't anywhere close to being a Democrat, and he throws out his share of red meat, but he seems to understand that the issues that Republican activists and conservative media outlets fixate on don't resonate outside of the roughly third of the country having those conversations. He backs President Obama's Cuba rapprochement, for example, and he uses a Teleprompter, because it is a useful tool for reading speeches in public — and not one Obama was the first to make use of.

The same things that make Paul potentially attractive to independents and certain left-leaning constituencies, though, make him suspect to the only voters that can make him the GOP presidential candidate.

As hard as he's worked to build bridges to the GOP establishment — he all but has the endorsement of Senate Majority Leader Mitch McConnell, for instance — lots of Republicans still associate Paul with the more heterodox views of his father, former Rep. Ron Paul (R-Texas). On the other hand, Paul's concessions to his party's establishment and base don't sit well with the motivated group of libertarian conservatives who made Ron Paul a force to be reckoned with in the 2008 and 2012 GOP primaries.

Then there's the political culture of the Republican Party, which hasn't shown much appetite to put new or untested faces on the top of its presidential tickets: Mitt Romney, John McCain, George W. Bush, Bob Dole, George H.W. Bush, Ronald Reagan, Gerald Ford, Richard Nixon. The only outlier there is Ford, a congressman who happened into being the incumbent in 1976. The last real risk the GOP took was Barry Goldwater in 1964, and he lost in a landslide.

Rand Paul will have money to compete, and he will have a grassroots network to tap into, but he won't win the 2016 Republican nomination. Hillary Clinton, or the Democrats' Candidate X, should count her blessings.