Donald Trump has given us a lot of firsts.

He was the first to green light the slaughter of our allies.

He was the first president to use Twitter to make national and international policy and fire senior staffers and recap Fox News shows.

He was the first president to pay hush money to a porn star.

Last week, he was the first to get applause at a rally for suggesting we build a border wall — along the border of Colorado.

The list could go on and on.

All of those will barely be a blip in history if he can pull off his biggest feat: Become the first president to be impeached and then go on to win reelection.

That should scare the life out of sane people. But, as ridiculous as it sounds, it is a very real possibility.

First, the history. Andrew Johnson was the first president impeached. Senators fell one vote short of expelling him, Johnson ran for reelection and lost his party’s nomination.

Richard Nixon, of course, resigned before the House voted to impeach him during his second term.

Bill Clinton was impeached by the House in 1998, was acquitted by the Senate and, although his popularity reached its peak amid the impeachment hearings, he was in his second term and couldn’t run again.

Which brings us to Trump.

The testimony of William Taylor, the U.S. envoy to Ukraine, last week, was about as damning as it gets. Taylor laid out a detailed timeline of a clear quid pro quo that made U.S. aid contingent upon Ukrainian officials finding dirt on Democrat Joe Biden.

No amount of clownish hearing-storming disruptive antics or boot-licking cable news appearances by Rep. Chris Stewart can undo the damage. Impeachment seems almost certain. And it’s equally certain that Sen. Mitch McConnell and Republicans in the Senate will put party first and let their president off the hook.

That sets the stage for the 2020 election. If you look at national polls, Democrats appear strong. Biden leads by 10 or 11, Sen. Elizabeth Warren leads by 8-10, and Sen. Bernie Sanders leads by as much as 9 in head-to-head matchups with Trump.

But national polls don’t matter. The election comes down to a few swing states, where the latest polling shows a tight race.

• In Ohio, perhaps the swingingest swing state, Biden is ahead of Trump by 2 while Warren and Sanders are tied.

The takeaway: Biden leads in the most states, but is no longer the clear Democratic front-runner. Warren and Sanders are within the margin-of-error, making basically every state that will decide the presidency a toss-up.

Trump’s approval, meanwhile, has only dropped slightly. It’s roughly the same as Barack Obama’s a year before winning reelection, and remains a whopping 87% among Republicans. The betting markets, despite a recent dip, still give Trump a 40% chance of being reelected.

Even in Utah, a poll released last week showed that Trump’s approval was 53%, up 5 percentage points from a survey conducted by the same pollster three months earlier — despite, or possibly because of, looming impeachment.

I think it boils down to two things.

First, Trump isn’t a president. He’s a cult leader. He could tell his followers to shave their heads, castrate themselves, drink poisoned Kool-aid and light themselves on fire and nearly 9 out of 10 Republicans would do it — and Sen. Pierre Delecto would tweet that he was “deeply troubled” by the recent developments.

If it means tax cuts and a couple more Supreme Court justices, most of them would gladly vote for Pennywise — the demonic clown from “It”.

The second problem, though, is that Democrats are trapped in a destructive nominating process. Instead of debating how to heal a divided country, it’s a fight to see who can promise more free stuff and take away guns. It plays right into the Republican socialist boogeyman narrative and alienates those who could be swayed and turn this election from a nail-biter into a landslide.

The candidate who focused the most on appealing to swing voters was Biden, and he’s steadily slipped in the polls. It’s a terrible system that yields terrible results.

Look at recent history. Bill Clinton won on middle-class economic issues and promising average working people a fair shake — not big government. Obama won by exciting young people with an aspirational promise of a unified America.

Al Gore and John Kerry and Hillary Clinton lost because they were standard-issue Democrats who failed to connect outside of the base or mobilize moderate, working-class voters.

We know Trump’s playbook. His only mode is attack. He’ll fight like a caged wolverine, fire up his followers and sow more chaos than the Joker, cackling all the while.

All Democrats should have to do is offer up an alternative who is reasonable, competent, with a vision for a better country and is palatable to voters in battleground states desperate for an alternative — and, so far, they have failed.

Maybe the eventual nominee, whoever it is, can navigate back to the middle after the primaries. If he or she does not, or if the damage is already done, 2020 will come down to the wire and could give us the terrifying prospect of four more years of a Trump White House — and that’s something we can’t afford.