He stressed that he has never wanted to be proven wrong so badly in all his life, but the reality was we all needed to get used to hearing the words President Trump.
Moore already caused a stir with similar comments made last week.
In a Wednesday night appearance on an online edition of HBO's Real Time with Bill Maher, Moore said he thought the verbal attacks on Democratic presidential candidate Hillary Clinton at the Republican National Convention played to "a lot of people" Trump has to win over to become president.
"I think Trump is going to win. I'm sorry," he said.
"People are in denial of this, but the chance of winning is really, really good."
The director of Bowling for Columbine and Fahrenheit 911 compared Trump's strategy to the successful campaign for Britain to leave the European Union, Associated Press reported.
He repeated such comments in his open letter released overnight where he writes: "This wretched, ignorant, dangerous part-time clown and fulltime sociopath is going to be our next president."
"President Trump. Go ahead and say the words, cause you'll be saying them for the next four years."
Moore also warns those who laugh yet are appalled by Trump need to get real because Hillary Clinton would unfortunately not become the first female President of the United States.
"You need to exit that bubble right now," he continues. "You need to stop living in denial and face the truth which you know deep down is very, very real."
He also writes how despite even people in his own party trying to stop Trump's rise, the fact that he won 56 primaries and caucuses against 16 candidates proved it is a genuine reality.
Moore then goes on to list the reasons why Trump will win the November election, ranging from a dislike of Hillary Clinton to feelings of voter disenfranchisement.
1. Midwest maths
Calling it "our Rust Belt Brexit" Moore said Trump is going to focus much of his attention on the four blue states of Michigan, Ohio, Pennsylvania and Wisconsin. Previously in favour of the Democrats, these have turned pro-Republic in recent years.
He warns what will happen in the UK will happen in the US with people felt abandoned by (Democrat) policies which helped lead to job losses and created a protectionist-type outlook. These people feel "screwed over" and while working classes might not agree with him they will end up voting for him because they are simply angry.
According to Moore, all Trump needs to win is 64 simple votes from this "rust belt" which will carry him over the line.
2. Angry white men
Moore warns the 240-year dominance of the US is coming to an end and the "endangered white male" is not happy about it.
Trump's argument fearmongering about the "Feminazi" is winning and the endangered white male will not put up with "a woman bossing us around" after having eight years of being told what to do by a black man.
"After that it'll be eight years of the gays in the White House! Then the transgenders!," he writes. "You can see where this is going. By then animals will have been granted human rights and a f****** hamster is going to be running the country. This has to stop."
3. The Hillary problem
Moore writes Clinton's biggest problem is the fact she remains so unpopular, with 70 per cent of voters finding her untrustworthy.
He adds another problem is she represents the old way of politics "not really believing in anything other than what can get you elected".
Clinton, he maintains, remains massively unpopular with young people and if anything can get voters to turn up to the polls it will unfortunately be Trump.
4. The depressed Sanders vote
Polls show Sanders voters will pick Hillary but that isn't the problem according to Moore. What is, however, is the fact that the average Bernie backer will only be voting for her reluctantly and not convincing five of their mates to do the same thing.
He also reveals how Clinton's killing the youth vote and few are excited or willing to volunteer to support her campaign.
"Hillary Clinton is going to have to do something to give them a reason to support her - and picking a moderate, bland-o, middle of the road old white guy as her running mate is not the kind of edgy move that tells millenials that their vote is important to Hillary," Moore warns.
The Jesse Ventura effect
Moore writes the voting booth remains one of the few places where there are no security cameras and where people can create anarchy.
"You can take as long as you need in there and no one can make you do anything," Moore writes. "You can push the button and vote a straight party line, or you can write in mickey mouse and Donald Duck. There are no rules."
He said it is because of this that millions will vote for Trump not because they like him but simply because they can and they feel disenfranchised and will wonder what a world with Trump will look like.
He warned this could happen after the "smart state of Minnesota" voted for ex-wrestler and governor Jesse Ventura in 1999.
"They didn't do this because they're stupid or thought that Jesse Ventura was some sort of statesman or political intellectual," Moore writes. "They did so just because they could."