Hillary Clinton has hired eight “summer fellows” as full-time paid staffers, just weeks after the Guardian revealed that the presidential frontrunner’s campaign was using experienced political operatives as unpaid interns.

The Clinton campaign is adding 20 full-time field organizers in Iowa, seeking to increase her presence and rally Democrats in the crucial caucus state, which she visited on Tuesday as her team acknowledged fears of a better-than-expected showing from the independent challenger Bernie Sanders.

As first reported by the Des Moines Register, eight of those new hires had previously served as full-time “organizing fellows”. In that capacity, fellows performed almost identical duties to field organizers, who do the most basic work on a campaign: recruiting volunteers and endorsements, going door-to-door and calling voters, often outside the confines of a 9-to-5 day job.

Now, those people will be paid, the Clinton campaign confirmed to the Guardian on Tuesday.

Clinton already had a sizeable footprint on the ground in Iowa and launched her campaign with roughly 40 staffers in the state, but the new wave of hires represents a decisive step by Clinton to build up her organizing efforts in the state.

The former secretary of state returned to Iowa on Tuesday for the first time since the opening week of her campaign, speaking to supporters and organizers in Iowa City ahead of her first sit-down interview since announcing her second run for the White House.

The push to woo ardent Iowa Democrats comes amid a surge of progressive appeal from Sanders, the Vermont senator who has been surging in recent polls and more than doubled his support among Democrats in the state between May and June, according to a Quinnipiac University’s survey.

“We are worried about him,” the Clinton communications director, Jennifer Palmieri, said of Sanders on MSNBC on Monday. “He will be a serious force for the campaign, and I don’t think that will diminish.”

The Iowa hires, however, represent the continuation of a trend in Clintonland that new staffers – no matter how much political experience they may bring from previous campaigns – initially must work for free before getting on the Clinton payroll.

Many high-ranking staffers worked for free as “volunteers” on the Clinton campaign in the weeks before she formally launched her campaign in early April. Political watchers told the Guardian last month it represented an attempt to wiggle around federal campaign-finance rules that ban an undeclared campaign from paying staff.

When Federal Election Commission reports are released next week, Clinton’s donations are expected to far exceed any other Democratic candidate.

Clinton’s competitors have already taken jabs at her use of the unpaid fellows: Lis Smith, the deputy campaign manager for former Maryland governor Martin O’Malley, tweeted over the Independence Day weekend that “we all know she doesn’t pay young staff”.

Still, Clinton has been polling well above 50% in the state where she finished third in the 2008 Democratic caucus.