(CNN) President Donald Trump insists that millions of illegal votes were cast during the election, after losing the popular vote to Hillary Clinton by nearly 3 million votes.

Trump has made allegations about voter fraud several times , and he's calling for a "major investigation" into his baseless claims -- claims he's been making since before last year's Election Day.

Of course there is large scale voter fraud happening on and before election day. Why do Republican leaders deny what is going on? So naive!

On Wednesday, Trump wrote in two consecutive tweets: "I will be asking for a major investigation into VOTER FRAUD, including those registered to vote in two states, those who are illegal and ... even, those registered to vote who are dead (and many for a long time). Depending on results, we will strengthen up voting procedures!"

Trump has made comments that three to five million illegal votes cost him the popular vote, according to two sources. Neither Trump nor his surrogates have provided evidence that any substantial illegal voting had occurred or influenced the popular vote.

Here's a closer look into voter fraud and the controversies around this issue.

Who can vote in the US?

Here's the criteria:

Be a US citizen

Meet your state's residency requirements (these vary depending on state)

Must be 18 years old on or before Election Day

Registered to vote by your state's registration deadline

What is voter fraud?

Voter fraud can be many things. Examples include:

Paying people to vote for a certain candidate

Voting multiple times in a federal election or impersonating a voter

Registering to vote or voting when a person is not eligible because of felony (this depends on the state) or lack of US citizenship

Lying on a voter registration card with a false name, address or period of residence

Intimidating voters physically or economically

Malfeasance by election officials

Voter fraud is a federal election crime that can be reported to a local election office, FBI, US attorney's office or the Department of Justice.

Are Trump's allegations actually voter fraud?

Here's what Trump's alleging in his tweets:

1. People registered to vote in two states

While it's illegal to cast ballots in multiple states, it's not illegal to be registered in two states at the same time.

Shortly after Trump's tweet about the need for a "major investigation" into this issue, one of his nominees and his chief strategist were found to have registered to vote in multiple states.

I will be asking for a major investigation into VOTER FRAUD, including those registered to vote in two states, those who are illegal and.... — Donald J. Trump (@realDonaldTrump) January 25, 2017

even, those registered to vote who are dead (and many for a long time). Depending on results, we will strengthen up voting procedures! — Donald J. Trump (@realDonaldTrump) January 25, 2017

Steven Mnuchin, Trump's nominee to head the US Treasury Department, is registered to vote in New York and California, a CNN KFile review found.

Steve Bannon, Trump's chief strategist and senior counselor, was r egistered to vote in both New York City as well as in Sarasota County, Florida. Bannon has since been removed from the voting rolls in Florida.

2. Non-citizens voted

This would be voter fraud, as only citizens can vote. But his claim remains unsubstantiated.

3. People voted in the names of dead people who are still on the voter registry

This too would be voter fraud, but remains unsubstantiated.

Does voter fraud exist?

A report called "The Truth About Voter Fraud" issued by the Brennan Center for Justice found voter fraud rates were between 0.00004% and 0.0009%.

Voter fraud exists "in very small numbers," said David Becker, executive director of the Center for Election Innovation & Research. He had directed a 2012 Pew Center study that underscored deficiencies in the voter registration system and which Trump has since made reference to.

JUST WATCHED Pew study director: Fraud just doesn't happen Replay More Videos ... MUST WATCH Pew study director: Fraud just doesn't happen 03:14

But he told CNN that "there is absolutely no evidence of voter fraud even approaching (the) scale" suggested by Trump.

"I agree completely with the secretaries of state across the country, both Republicans and Democrats, and the National Association of Secretaries of State which is bipartisan, which has said, there is absolutely no evidence of voter fraud even approaching that scale," he said.

The Pew study found that the American voting system was in need of an upgrade with millions of invalid registrations and dead people listed on the rolls. But the study did not find fraud.

Trump himself -- through his lawyers -- has also argued that there was no evidence of voter fraud in the 2016 election. In a court filing objecting to Green Party candidate Jill Stein's Michigan recount petition, lawyers for the president wrote , "All available evidence suggests that the 2016 general election was not tainted by fraud or mistake."

Effects on voting laws

Trump's insistence on voter fraud has Democrats fearing that this will morph into new efforts to limit voting access.

A total of 34 states have laws requesting or requiring voters to show some form of ID at the polls, according to the National Conference of State Legislatures.

This has been a raging battle between Republicans who say showing ID is a way to fight voter fraud and Democrats who say the true aim of such voting ID laws, cutbacks on early voting and same-day registration is to create a substantial burden on the right to vote, particularly for minorities.

JUST WATCHED Van Jones' theory on Trump, voter fraud Replay More Videos ... MUST WATCH Van Jones' theory on Trump, voter fraud 01:48

Some states require a photo ID, such as a driver's license, which supporters say are a simple and painless way to assure the integrity of the voting process.

But opponents say this disenfranchises poor and minority voters, who face difficulties obtaining IDs and documents. The NAACP has estimated that 25% of African-Americans and 16 perceent of Latinos of voting age lack a current government-issued photo ID.

Critics have likened the voter ID laws to the poll tax used in the late 1800s and early 1900s as a method to prevent blacks from voting.