She was an heiress without a cause — an indifferent student, an unhappy young bride, a miscast socialite. Her most enduring passion was for birds.

But Cordelia Scaife May eventually found her life’s purpose: curbing what she perceived as the lethal threat of overpopulation by trying to shut America’s doors to immigrants.

She believed the US was “being invaded on all fronts” by foreigners, who “breed like hamsters” and exhaust natural resources.

She thought that the border with Mexico should be sealed and that abortions on demand would contain the swelling masses in developing countries.

An heiress to the Mellon banking and industrial fortune with a half-billion dollars at her disposal, Ms May helped create what would become the modern anti-immigration movement.

She funded the nation’s three largest restrictionist groups — the Federation for American Immigration Reform, NumbersUSA and the Centre for Immigration Studies — as well as dozens of smaller ones, including some that have promulgated white nationalist views.

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Fourteen years after May’s death, her money remains the lifeblood of the anti-immigration movement, through her Colcom Foundation.

It has poured $180 million (£148 million) into a network of groups that spent decades agitating for policies now pursued by current US president Donald Trump: militarising the border, capping legal immigration, prioritising skills over family ties for entry and reducing access to public benefits for migrants.

“She would have fit in very fine in the current White House,” said George Zeidenstein, whose mainstream population-control group Ms May supported before she shifted to anti-immigration advocacy.

“She would have found a sympathetic ear with the present occupant.”

The New York Times has unearthed the most complete record of her thinking through reading her unpublished writings which reveal her evolution into an ardent nativist.

Chatty, handwritten notes about luncheons and overseas trips gradually gave way over the years to darker exchanges with fringe figures who believed that black people were less intelligent than white people, Latino immigrants were criminals and white Americans were being displaced.

But Ms May disputed the notion that she was racist, writing in 1994: “Can we not put imaginary paper bags over the immigrants’ heads, see them as colourless consumers, and count only their deleterious numbers?”

Restrictionist groups she financed have blocked immigration reform bills in congress over the years.

Trump attacked immigrants for 'murders, killings, murders' during most recent El Paso visit, months ahead of shooting

For example, they fought for Proposition 187 to deny education, routine health care and other public services to immigrants in the country without permission in California. They have argued against in-state tuition for the children of undocumented workers in Utah.

“We occupied the space before anybody, and the people who helped found the organisation and fund the organisation, including Ms May, were people of enormous foresight and wisdom,” said Dan Stein, president of the Federation for American Immigration Reform (FAIR).

“They would be gratified over the fact that we’ve seen these ideas championed at the highest level.”

FAIR said in a federal tax filing last year that Mr Trump’s election presented “a unique opportunity” to enact its long-standing agenda of “building the wall, ending chain migration, rolling back dangerous sanctuary policies, eliminating the visa lottery” and more.

“Without Cordy May, there’s no FAIR,” said Roger Conner, the organisations’ first executive director. “There was no money without her.”

Ms May started out at Planned Parenthood in Pittsburgh, later joining the board of the International Planned Parenthood Federation.

In her spare time, she flew around the world on nature expeditions.

Her twin passions, protecting natural habitats and helping women prevent unplanned pregnancies, merged over time into a single goal of preserving the environment by discouraging offspring altogether.

“The unwanted child is not the problem,” she would later write, “but, rather, the wanted one that society, for diverse cultural reasons, demands.”

Ms May joined the board of the Population Council, a group founded by John D Rockefeller III that emphasised family planning and economic development as ways to lower birthrates around the world.

One day, however, her top aide delivered a stern message to Mr Zeidenstein, the group’s new president, saying family planning and famine relief were a waste of money.

Instead: “The US should seal its border” with Mexico.

According to a memo by Mr Zeidenstein, Ms May’s views were becoming so radicalised that “one got the impression” she favoured compulsory sterilization to limit birthrates in developing countries.

She then moved onto funding the Environmental Fund, which pushed mainstream concerns about overpopulation to the fringe and stoked opposition to immigration.

She met John Tanton, a charismatic eye doctor and environmentalist from Michigan, who would leverage Ms May’s financial resources to propel the budding anti-immigration movement forward.

Immigration surged in 1978, when the Border Patrol apprehended 863,000 unauthorized immigrants, the most in over two decades. Another 601,000 legal immigrants also arrived.

Mr Tanton wrote a nine-page proposal that November asking for funding from May to start a group called the Federation for American Immigration Reform, or FAIR.

“We plan to make the restriction of immigration a legitimate position for thinking people,” he wrote. Ms May provided $50,000 to get the group off the ground.

FAIR’s early policy goals, some reflected decades later in proposals pursued by the Trump administration, called for not only an end to illegal immigration, but also a sharp reduction in legal migration.

The group advocated increased funding and staffing for Border Patrol to police the southern frontier, campaigned against Cuban refugees and pushed to restrict public benefits for undocumented immigrants.

In 1996, a 68-year-old Ms May established a new foundation, Colcom, to pursue her most important goals even after her death.

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According to tax documents, Colcom has funded not only FAIR and other large organisations May helped create, but also lesser-known ones like the American Immigration Control Foundation, which has likened immigration to a “military conquest” with the effect of “substantially replacing the native population”.

Colcom has also funded the International Services Assistance Fund, whose focus is promoting chemical sterilisation of women around the world, and VDare, a website that regularly publishes white nationalists.

Though her money and activism seeded the political landscape for Trump’s nativist policies — he argues that “the country is full,” claims Mexicans are “dirty” and “dangerous” and immigrants are stealing jobs — the heiress did not live to see him ascend to presidency.

Ms May, who had pancreatic cancer, died at her home in 2005, aged 76. Her death was ruled a suicide by asphyxiation.

Because of how she invested her vast inherited fortune, Ms May’s ideas and causes survive her.

“The issues which I have supported during my lifetime have not been popular ones in many cases, nor do I anticipate that they will be so in the future,” Ms May wrote to Colcom’s board members. calling on them “to exercise the courage of their convictions” after her death.