MIAMI — Nearly 15.3 million Americans tuned in on Wednesday for the first debate of the Democratic nominating contest for president, a higher number than NBC News producers had anticipated and a sign of significant national interest in the 2020 campaign.

The television viewership nearly equaled the record for a Democratic primary debate, set in October 2015 for a matchup on CNN that included Hillary Clinton and Senator Bernie Sanders of Vermont.

By contrast, Wednesday’s debate was arguably the less enticing half of a two-day event in Miami, with only one candidate, Senator Elizabeth Warren of Massachusetts, in the top polling tier of the current field.

The big audience — 495 days before Election Day and seven months before the Iowa caucuses — suggests that Americans are already deeply invested in the contest over who will challenge President Trump next year.