The 2016 presidential race has produced two polarizing presumptive candidates with high negative ratings who must go after the razor thin slice of "hardcore independent" voters, said Larry Sabato, founder and director at the Center for Politics at the University of Virginia.

The veteran political analyst told CNBC's "Squawk Box" on Friday he considers only 4 to 6 percent of voters as true independents. The other wildcard are "hidden partisans," voters who really do know whom they're going to be vote for but chose to keep their picks secret.

In the latest RealClearPolitics average of national polls, Democrat Hillary Clinton leads Republican Donald Trump by 3.8 percentage points in a head-to-head match, with 44.1 percent support versus 40.3 percent. Adding Libertarian nominee Gary Johnson, formerly a Republican governor of New Mexico, to the mix, Clinton's lead actually shrinks to 2.5 points over the real estate billionaire. The polling average shows Johnson drawing 9 percent support.

"Whatever happens on the third-party front, we're going to get one of the two major-party nominees, obviously, elected president," Sabato said.

While Clinton received 15.6 million voters in the nominating contests and Trump got 13.3 million, that's a "pond of voters compared to the ocean in the general election," Sabato said, adding that about 135 million people generally turn out for the main event.

With experts so wrong about Trump in the primaries, Sabato said general election contests, perhaps counterintuitively since there's such a larger pool of voters, are traditionally easier to call.

"[Americans] tend to respond to the same factors every four years: the popularity of the incumbent president; what they think about the incumbent president and his party; the state of the economy; what's happening in foreign affairs, particularly if there's something big going on; [and] big scandals. These are the same factors that pretty much produce every presidential result," he said.

Now that the presumptive presidential candidates for each party are known, the veepstakes guessing game about whom they might pick as running mates heats up.

Sabato predicts Trump would choose his vice presidential nominee during the Republican National Convention, to be held from July 18-21 in Cleveland.

"[Trump] is a showman. And he knows you have to build drama to build an audience," Sabato said. "He wants to turn the convention into a show that actually attracts a very large audience."

As for Clinton, Sabato believes the "process oriented" former secretary of state would choose her running mate ahead of the Democratic National Convention in Philadelphia, held just days after the GOP gathering, from July 25-28.

Sabato told the "Squawk Box" co-hosts: "As you guys know, because you cover the [financial] markets, it's always important to have as much good information as possible before you make a big decision. And knowing who Trump's VP pick is kind of a big piece of important information."

Massachusetts Sen. Elizabeth Warren, who's been mentioned as a possible Clinton VP pick, is meeting with the presumptive Democratic nominee in Washington on Friday morning.

Warren, a champion of the far-left, endorsed Clinton on Thursday night, following President Barack Obama's official endorsement of Clinton.