“We can never be defeated,” he said, his voice soaring and reverberating through the convention hall. “Even when we’re knocked down, we have to understand that we are never knocked out.”

It is a ritual of politicking for presidential hopefuls to swing through this early-voting state, whose reputation as a political bellwether draws an array of potential candidates — or anyone who hopes to be seen as one — long before any election is to take place. Eric Garcetti, the mayor of Los Angeles, spent two days here in April, talking to union carpenters, activists and firefighters. Michael Avenatti, the lawyer for Stormy Daniels and an anti-Trump crusader, stormed into the state in August to declare he might run for president in 2020.

Even lesser-known politicians have descended on the state. Representative John Delaney of Maryland has campaigned in all of Iowa’s 99 counties.

As the midterm elections approach, other possible 2020 candidates, including Senator Elizabeth Warren of Massachusetts and former Vice President Joseph R. Biden Jr., are also fanning out across the country — to campaign for midterm candidates but also to get themselves in front of voters. Senator Kamala Harris of California is in Ohio this weekend, where she is keynoting the Ohio Democratic Party’s state dinner.

Then there is Mr. Avenatti, who was in Ohio on Friday and in New Hampshire last weekend, lest anyone forget he might be running, too.

The flood of appearances is a sign of just how crowded the field of Democratic candidates could be heading into 2020, and how aggressively they will make the case that they are better than President Trump. Energized anew by a bitter Supreme Court confirmation process, many Democrats are looking for a leader who can offer a different vision for the country, and fight back against an administration they see as reckless, toxic and hostile.

Last weekend, Ms. Warren gave the strongest signal yet that she would be a candidate, saying she was taking “a hard look” at a 2020 run because the “broken government” needed to be fixed.

Mr. Booker’s trip to Iowa, his first since he campaigned in the state for Hillary Clinton in 2016, followed recent campaign stops in Texas, Georgia and Florida, all of which have tight races whose outcomes could shift the momentum for the Democrats. Mr. Booker, who has family in Iowa, will also be in the state on Monday for a get-out-the-vote rally. (His family is throwing him something of a family reunion on Sunday, according to an aide.)