If you want a reminder of what happened last night, here's a quick recap, courtesy of BBC reporters who were on the ground in Iowa.

Ironically, Pete Buttigieg has campaigned to scrap the electoral college and have the US national election decided by the popular vote.

“At risk of sounding a little simplistic, one thing I believe is that in an American Presidential election, the person who gets the most votes ought to be the person who wins," he said in a CNN town hall last year.

He is currently leading the results in Iowa on delegate numbers, but behind Bernie Sanders in the popular vote.

If the preliminary results bear out, and he wins Iowa but loses the popular vote to Sanders, he might not be rehearsing that argument in the very near future.

Image source, Chip Somodevilla/Getty Images

We've been focused on the Iowa results for much of the day but the Democratic horse race is not the only thing vying for Washington's attention this evening.

On Capitol Hill, Donald Trump's impeachment trial is still ongoing. Senators will convene tomorrow to vote on whether to convict or acquit the president on two charges: abuse of power and obstruction of Congress.

And in about two and a half hours - 21:00 local time - Trump will address the nation in the annual State of the Union address. We'll start to shift our coverage to the president's remarks tonight.

Trump will tout US economic and military strength, US media say.

There's a reason why both Pete Buttigieg and Bernie Sanders declared victory last night - and it's down to the numbers.

Bernie Sanders has the popular vote so far - with 28,220 votes for him to Pete Buttigieg's 27,030.

But Buttigieg has the highest percentage share of delegates - 26.8% to Sanders' 25.1%.

Ultimately, it's the delegates who go on to represent their voters' interests in later stages of the nominating process, by voting for them as the party's nominee at the Democratic convention in July.

But Iowa's significance comes mostly from the fact it comes first in the race, rather than the number of delegates it has up for grabs (41). So candidates will be trying to gain momentum, and spinning the results as a victory, however possible.

In early December, we looked at Joe Biden's tour of Iowa, headlined 'No Malarkey.' Although he has his loyalists, it was clear even then that not everyone in Iowa was full of enthusiasm for the former vice-president. It's worth looking at the reasons they cite - could these factors play out in other states?

Nick Bryant
BBC News, Des Moines

Joe Biden made electability central to his campaign, making the case he is able to beat Donald Trump in November's election – but he seems to have performed very poorly.

Pete Buttigieg is going to get a big bump out of this, although perhaps not as much as if results were declared last night.

We have not been given clarity from the Iowa Democratic Party over when we will get final results. But one thing’s pretty clear here – Joe Biden has had a pretty bad time. He will argue Iowa was never friendly, and he’ll go on to places like South Carolina – he is very popular with black candidates.

But Iowa has a habit of spotting candidates who are underperforming. During his stump speech, many people were shocked by Biden's incomplete thoughts and lack of energy.

Still, it’s very early days – we don’t even know the final results in Iowa yet – and it’s one contest with many more to come.

It's worth pointing out, amid all the fanfare over the Iowa results, that the 41 delegates awarded by the state represent just 1% of the national total.

With just 62% of results reported in Iowa, this state could even swing to a different candidate at this stage. And Joe Biden will take solace in the face that there's still a long way to go to the Democratic convention.

Anthony Zurcher
BBC North America reporter

We finally have some 2020 Iowa caucuses results to talk about. And they’re going to generate a lot of talk – and hand-wringing.

There are clear winners, as Bernie Sanders and Pete Buttigieg can both claim some kind of victory, depending on how the final tabulations come in.

There’s also a clear loser – Joe Biden. He entered Monday leading in some polls and hoping for a strong showing that would put to bed concerns that he is a flawed front-runner.

Instead those concerns are wide awake and pacing the room.

Unlike candidates like Sanders, Elizabeth Warren and Buttigieg, Biden’s campaign coffers are thin – and this Iowa performance isn’t opening any cash spigots.

It could have been worse for the former vice-president – he could have had to give a fourth-place concession speech on Monday night – but the result is the same. He was unable to land a knock-out blow on fellow moderate Amy Klobuchar, while Buttigieg and Sanders both exit Iowa strengthened, suggesting Biden could be staring at a third-place finish – or worse - in New Hampshire.

Until proven otherwise, Biden still has his southern-state firewall, based on support from elderly and black voters. But the flames are licking at his heels.

"We got first place!" said Pete Buttigieg, addressing supporters in New Hampshire, though the final results are not yet in.

He said his campaign had "taken its place at the front of this race to replace the current president with a better vision for the future".

He called his early lead a "validation of this campaign, this candidate, and this vision you've all been a part of".

"It's a validation of the idea that we can expand a coalition not only unified by who it is we are against but by what it is we are for," he said.

"And it validates for a kid somewhere in a community wondering if he belongs or she belongs or they belong."

The man who may win Iowa is speaking at a press conference in New Hampshire. We'll bring you his comments soon.

Image source, Getty Images

Who? Became a city mayor when still in his 20s and served in the Navy, first openly gay candidate

Key issues: Political reform; LGBTQ rights; college loan relief

One policy: Buttigieg has proposed reshaping the Supreme Court to have five Democratic appointees, five Republican, and five selected by an agreement of the 10 appointed justices

Read more, including analysis from the BBC’s Anthony Zurcher here.

The results reported tonight are a good reflection of the PR approaches taken by the various campaigns last night, while the public was still in the dark as to how any of the candidates had performed but their campaigns were working off preliminary information.

The Buttigieg and Sanders campaigns both claimed victory - and the results tonight show Buttigieg with the most delegates and Sanders with the highest popular vote total.

Biden's campaign came out last night in attack mode, foreshadowing a poor performance by openly questioning the integrity of the caucus.

Warren in her speech looked forward to the states to follow, indicating she was also not expecting to be able to boast about the results in Iowa.

With only partial results so far, our team here in Washington are wondering whether it would have been better if Iowa's Democrats had just waited for the full results.

Based on current results, Pete Buttigieg is doing better than expected - while it's being seen as a bad night for Joe Biden.

Pete Buttigieg is leading the caucus results with the most delegates, but Bernie Sanders is ahead in the popular vote count with 28,220 votes to Buttigieg’s 27,030, according to totals reported by CNN.

Elizabeth Warren has 22,254 votes and Joe Biden 14,176, with 62% of precincts reporting.

If this stays the same, Joe Biden is in what will be seen as "a very disappointing fourth place", says BBC correspondent Nick Bryant.

"The most explicit claim of victory came from Mayor Pete - these early figures do suggest that Pete Buttigieg is leading at the moment, so it may be understandable why he was so confident of victory late last night," our correspondent adds.

Here's the breakdown of that first 62% of results from Iowa Democrats, showing the percentage share of delegates won so far:

• 26.9% – Buttigieg
• 25.1% – Sanders
• 18.3% – Warren
• 15.6% – Biden
• 12.6% – Klobuchar
• 1.1% – Yang
• And less than 1% for Tom Steyer and Tulsi Gabbard

After a process mired in chaos, Pete Buttigieg has a narrow early lead in the Iowa caucuses, partial results show according to CNN. He has an estimated 26.9% of delegates with 62% of results counted.

Troy Price, Iowa Democratic Party chairman, is now speaking. He called the circumstances last night "unacceptable", and said the party would report 62% of results in a few minutes.

"We were faced with multiple reporting challenges," Price said.

"But let me be clear, my number one priority is ensuring the accuracy and integrity of the results. We have been working all night to be in the best position to report the results.

"The bottom line is, we hit a stumbling block on the back end of the data, but the one thing I want you to know is this data is accurate

The Iowa Democratic Party said in a statement that chairman Troy Price would address reporters at 15:45 local time (20:45 GMT). It's running a little late.

Zoe Thomas
US business reporter

Shadow Inc, the firm behind the app that caused the delay in reporting the Iowa caucus results, apologised in a Twitter thread on Tuesday.

The Democratic National Committee (DNC) and Shadow both said the problem was a coding error, stressing that results were not corrupted.

There were problems with the app before the caucus even began. Members of the Iowa Democratic party reported issues downloading and logging into the app, leaving some concerned it was not ready for the big night.

The state of Nevada said it would not use the same technology in its primaries on 22 February and had already developed redundancy systems for reporting results.

Some security experts have already issued warnings about the use of technology in the voting process - saying that could create an opening for election interference.

Derek Eadon, a former Iowa Democratic Party chairman called the issue "a systemwide disaster".