Four million. That's how many Americans the US Census Bureau estimates live on five island territories of the United States.

Millions of them are of voting age. Many are veterans or active military. But they cannot vote to elect their commander in chief.

People born in Guam, the Virgin Islands, the Northern Mariana Islands, American Samoa, and Puerto Rico are all Americans. They vote in US congressional elections and presidential primaries. This year, voting rights advocate and lawyer Neil Weare says they were "even heavily courted by both parties ... they went to the [Democratic and Republican] conventions."

But Americans born in these territories can't vote for president. Not unless they move to the mainland.

This is in spite of the fact that they're all US citizens (except for American Samoans, who are only US nationals. More on that in a moment.)

So, why is this — why can't all US citizens vote in general elections? The short answer: Because the Electoral College says so. Bryan Whitener, spokesperson for the Election Assistance Commission, a government agency, quoted the National Archives on the matter:

"No, the Electoral College system does not provide for residents of U.S. Territories (Puerto Rico, Guam, the U.S. Virgin Islands, Northern Mariana Islands, American Samoa, and the U.S. Minor Outlying Islands) to vote for President. Unless citizens in U.S. Territories have official residency (domicile) in a U.S. State or the District of Columbia (and vote by absentee ballot or travel to their State to vote), they cannot vote in the presidential election. Note that prior to the adoption of the 23rd Amendment, DC residents could not vote in the Presidential election. The political parties may authorize voters in primary elections in Territories to select delegates to represent them at the political party conventions. But that process does not affect the Electoral College system."

These rules are why pollsters and news outlets have been carefully watching a mass movement of Puerto Ricans to the US mainland in recent years. Once they establish residency in a state, these US citizens can vote in the presidential election — and potentially affect the outcome.

Weare, who heads the We The People advocacy group, doesn't think US citizens should have to relocate in order to vote for their president. His DC-based group is involved in lawsuits to grant voting rights to Americans in territories — to do for them what the 23rd Amendment did for natives of Washington, DC in 1961.

"The US has had territories since day one," he says, "but they've always gone on to become states. It's only with the acquisition [of] the overseas territories that we can keep these places but never really have them be part of the political community."

Weare thinks the current voting laws are especially unfair in Guam and American Samoa, given their high military enrollment. In 2014, the Army said American Samoa was the United States' No. 1 military recruitment post.

"Someone who's denied full participation in American democracy," Weare says, "for these people who've served to defend the American Constitution, it's a real insult to them as Americans."

John Oliver made the same point last year on his weekly HBO show, "Last Week Tonight":

Among permanently inhabited US territories, the general election rules are weirdest for American Samoa.

Like other territory residents, American Samoans can vote in US primaries but not for presidents. But they can't do what millions of Puerto Ricans have been doing: relocate to the mainland permanently like a person would move from state to state. That's because American Samoans, unlike other territory residents, are not citizens. (Check out this article for more on American Samoans' quest for citizenship.)

If they want to move to a US state or Washington, DC, they have to go through an immigration process similar to that for people from other countries. They have to become naturalized. Naturalization costs almost $700 in fees and includes a test of the English language, US history and civics.

Weare calls this the equivalent of a literacy test and poll tax — something that was outlawed by the 24th Amendment to the Constitution.

Chief Loa Pele Faletogo is one of the plaintiffs in a case demanding full citizenship for American Samoans in California, a state where many American Samoans have settled.

"There's a whole lot of seniors and a lot of people who want to vote, and they can't," he says. "And No. 1, there's a language barrier also ... and they're surviving on whatever [Social Security Income] they have, instead of spending all the money to pay for the citizenship test."

Faletogo, who heads the Samoan Federation of America, got naturalized through the military, a common pathway.

"They've done so much for these States," he says. "Died in the wars — their sons and daughters."

Come Nov. 8, Faletogo expects his fellow naturalized American Samoans will vote Democratic "because they're the ones preaching right now that they're trying to do something [for us]." Many expect mainland Puerto Ricans will do the same.