Donald Trump. AP Now that Donald Trump has effectively clinched the Republican nomination, many Democrats are exchanging high-fives.

Trump's victory, they believe, will guarantee a Hillary Clinton win in November and secure the White House for the Democrats for another four years.

Most of the pundits who declared Trump's candidacy dead on arrival last summer also believe this.

Democrats and pundits are confident because during his scorched-earth primary campaign, Trump repeatedly said things that would make almost anyone else unelectable to national office — including frequently insulting and offending vast groups of voters such as women and minorities.

Their confidence is not illogical. Trump starts the general election with a 6.5-point average deficit in polls. And he is the most unpopular major-party nominee ever.

But those who believe the Trump nomination assures a Clinton victory are not giving enough weight to two important facts:

1. Hillary Clinton is a weak campaigner; Trump is a strong one. At its core, campaigning is about selling a product — the candidate. The reason Trump clobbered the Republican field is that he's a great salesman. Hillary Clinton is not. By her own admission, Clinton is much better at actually doing political jobs than campaigning to get them.

2. No one really knows what Trump believes or would do as president, but we do know that he'll say whatever he thinks will give him the best chance of winning. So far, Trump's pitch has been tailored to appeal to one particular voter constituency — the angry, fed-up right wing. Now that Trump has won over that constituency, he'll turn his sales efforts, and image, to winning another: the center.

A great salesperson figures out what customers want — and then persuades them that, if they buy the product, they'll get what they want.

In the Republican primary, Trump figured out what Republican voters wanted. And he persuaded enough of them that, if they chose him, they would get it.

Most Democrats still think that Trump will get crushed in the general election because he actually believes what he was selling in the Republican primaries — the plan to round up and deport all of the immigrants living in the US illegally, for example. Or the plan to bar Muslims from entering the US.

Donald Trump in his office with Business Insider founder Henry Blodget. Sam Rega/Business Insider

It's possible that Trump does believe these things and intends to act on them as president.

But to me it seems more likely that Trump was just "projecting an image," as his team reportedly suggested a couple of weeks ago to Republican leaders when speaking behind closed doors.

When Business Insider executive editor Brett LoGiurato and I interviewed Trump in his office last fall, he came off as more reasonable and practical than he does in his public appearances. When we noted the absurdity of rounding up 11 million people and shipping them out of the country, for example, Trump hesitated in responding.

Trump didn't actually say it, but what I expected him to say after this hesitation was this:

Come on, Henry. I'm a practical guy. I know the idea of deporting 11 million people is ridiculous. But I also know that it's what Republican voters want to hear. Let me win the nomination and election, then we'll study the situation. And then I'll fix it! Just the way I fixed Wollman Rink!

Now that Trump has won his first sales pitch — the nomination — he, like any great salesperson, will adjust his pitch to win the next one: the general election.

He won't disavow any of the hateful things he said to win the Republican primary. He'll just ignore them. And instead of pitching to angry right-wing voters, he'll start pitching to the vast numbers of more centrist Americans who just aren't all that stoked about voting for Hillary Clinton.

And in that sales pitch, Trump will have another advantage: He's selling something positive.

Specifically, Trump is promising to "make America great again."

Hillary Clinton is already responding by selling something negative: "Prevent President Trump."

Democratic presidential candidate Hillary Clinton addresses the attendees of the 61st Annual NAACP 'Fight For Freedom Fund Dinner' in Detroit Thomson Reuters

The "block Trump" pitch may scare some people into voting for Clinton, but it's not as compelling a pitch as "making America great again" — especially when, for so many Americans, on both the right and left, America's economy is clearly not that great.

So Democrats who are viewing Trump's victory as a guarantee that the White House will remain in their hands might want to rethink that.

The next six months will be about selling two candidates to about 51% of the American public.

Donald Trump is a great salesman.

Hillary Clinton is not.

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