This week marks 25 years since Bill Clinton signed the Family and Medical Leave Act, which gives US workers the right to unpaid time off to care for themselves and close family members.

It took another decade for some to win paid sick leave, when San Franciscans approved a ballot initiative in 2006 for private employees to earn an hour of paid sick time for every 30 hours worked. Similar measures now benefit 14 million workers in 32 municipalities and nine states.

Paid sick leave advocates cite studies showing flu infection rates decrease in cities where workers earn sick days, and that parents who cannot take leave are two times more likely to send their sick children to school. They also point to a 2012 poll of restaurant servers and cooks that revealed two-thirds had served or cooked food while ill, threatening the health of their co-workers, customers and the companies that employ them.

But with a flu epidemic currently raging across the US, potential new sick leave measures are facing opposition from the same Koch Brothers-backed lobbying group that led the legal assault on Obamacare.

When Maryland lawmakers moved last month to override the governor’s veto of a bill allowing 700,000 workers to earn sick leave, the state’s director of the National Federation of Independent Business – the Koch-backed group – complained it would create job-killing costs and mandate “devastating sanctions” for failure to comply.

On Thursday, the NFIB backed a failed attempt to delay the law, which went into effect on Sunday.

Now it wants Austin city council members to vote no this Thursday on an ordinance that would make the Texas liberal enclave the first city in the south to require paid sick leave from private employers.

Past tax records reveal most of the NFIB’s funding comes from Freedom Partners, whose nine-member board includes eight current or former key figures at Koch Industries and other Koch entities. More than 95% of the candidates it backs are Republican.

While its representatives are often quoted in the media as proponents of small businesses, the group refuses to release its donor list and tends to lobby for policies that benefit billionaires and corporate interests.

Under the Austin plan, employers would be able to ask for verification when sick leave extends beyond three days of work, are not required to pay out sick time when a worker quits, and can cap the number of sick days at eight per year.

But the NFIB’s call to vote no uses generic talking points from other lobbying campaigns to wrongly claim the ordinance “has no provision for reasonable notice to employers regarding the employee’s absence” and “does not set a limit on accrued time”.

Upping the stakes, NFIB’s Texas Executive Director, Will Newton, has promised to pursue state-level a “pre-emptive” law against paid sick leave next year. Seven states have already enacted similar measures drafted by the American Legislative Exchange Council, which critics call a corporate bill mill.

This comes as employers in areas that now require paid sick leave have expressed support for the policy, after first complaining lawmakers lacked a clear understanding of business operations.

A University of Washington survey of local business owners about the impact of Seattle’s 2011 ordinance found their “initial fears had faded” and instead of prompting an exodus, “the number of employers grew more in Seattle than in comparison cities”.

In Connecticut, which became the first state to require employer-paid sick days in 2012, a 2014 study found 10% of small businesses reported the law increased payroll costs by 3% or more, with the average worker taking just four sick days, and half using three days or less.

Meanwhile, during a press conference on Tuesday to announce the death of two children from the flu this season in New York City, the health commissioner touted the city’s paid sick leave law as a way to get people with the virus to stay home.

The measure passed in 2014, but a report released last month found fewer than half of low-income workers had heard of it.