Trump has already hired two political operatives to be based in Iowa full time. One of them is the son of the former Republican Governor of Iowa Terry Branstad, who was also Trump's ambassador to China.
The operatives' main job is to keep an eye on other candidates.
Those who have already turned up in Iowa, to raise money and make speeches, include Trump's former Vice President Mike Pence, his former UN Ambassador Nikki Haley, his former secretary of state Mike Pompeo, former presidential candidate Senator Marco Rubio, South Dakota governor Kristi Noem, and the Republican senators Tim Scott, Tom Cotton and Josh Hawley.
One notable absentee from Iowa so far is Ron DeSantis, the Republican governor of Florida.
DeSantis has topped a series of "no-Trump" polls, including in Iowa, conducted on the assumption that the former president does not declare.
However, DeSantis, 43, would be unlikely to run against Trump, who he strongly supports, and said this week he will seek re-election as Florida governor next year.
Ms Haley, who had been touted as the most prominent moderate alternative to Trump, has publicly committed to not running if the former president does, saying she will support Trump if he runs.
Currently aged 75, Trump would be 78 years and five months old at the next election. That is six months older than Joe Biden was when he became the oldest person ever elected president in 2020.
However, Trump remains the overwhelmingly dominant figure in the Republican Party, in Iowa as everywhere else.
As would-be candidates seek to read the 2024 Republican electorate, polls by the Des Moines Register, the main newspaper in Iowa, have taken on an outsized importance.
J. Ann Selzer, the pollster who conducted the one this week showing growing support for Trump, said it appeared that "absence makes the heart grow fonder".
Among the many statistics that emerged was one that showed 86 per cent of unvaccinated people in Iowa had a positive view of Trump.
Just over half of Iowans are vaccinated against Covid-19. In Des Moines, supporters began camping out on Thursday night for prime spots at Trump's rally 48 hours later.
Richard Snowden, from Delaware, one of a group called the "Front Row Joes," was attending his 78th Trump rally since 2015.
He told local television: "It's a great way that we can give back to Trump, show him support, and give him back certain love, because he has, sadly, had so much hate directed towards him."
Jerry Steward, 81, a respondent in the Des Moines Register poll, said he continued to support Trump because he was "not a politician". He said: "I don't know what he is, but he's not that. The situation we have now is very bad, and he will be able to correct that situation quicker than anybody else."
At his rally Trump was due to be joined by Chuck Grassley, the 88-year-old US Senator from Iowa who plans to run for an eighth term next year, along with the popular Republican Iowa governor Kim Reynolds.
Biden's national approval rating this week plummeted to a new low of 38 per cent in a Quinnipiac poll. As the president struggles Trump has upped his fundraising activity, raising millions of dollars from dinners, golf tournaments and emails to supporters, and increased his media appearances.
He is believed to have considered officially declaring his 2024 candidacy in the wake of Biden's Afghanistan withdrawal debacle at the end of August, but was persuaded to wait by advisers. One senior Republican who spoke to him at the time said he had been "about ready to announce".
