The American Civil Liberties Union has said it is awash with donations and new members as it faces up to Donald Trump over the extent of his constitutional authority, having received nearly $80m (£64m) in online contributions since the election last November.

That number includes a record $24m surge over the two days after Trump issued an executive order banning people from seven Muslim-majority countries from entering the US. The ACLU said its membership had more than doubled over the past three months to nearly 1.2 million, a record figure, while its Twitter following had tripled.

The ACLU executive director, Anthony Romero, said “it feels like we’re drinking from a fire hydrant”, adding that the election had brought immigration, refugee, reproductive, civil and voting rights “to a high boil”.

“What’s really heartening is people are paying attention. They’re aware of the crisis on the horizon,” he said. “There’s a real sense of urgency.”

After Trump was elected, the ACLU greeted his presidency on its website and magazine with the words “see you in court”, the same expression Trump used in response to a federal appeals court decision refusing to reinstate the travel ban.

The ACLU has won court orders in New York, Massachusetts and Maryland against the travel ban. It has also filed a Freedom of Information Act request for documents on the billionaire’s potential conflicts of interest.

The organisation intends to bring a legal challenge accusing the president of violating the US constitution’s emoluments clause by accepting payments from foreign governments at his hotels and other properties.

The boost to the ACLU’s $220m budget from donations will allow it to spend more on state operations, which Romero said became critical after some legislatures took Trump’s election as a licence to promote anti-immigrant, anti-civil rights and anti-abortion legislation.

The 1,150-employee ACLU plans to hire more lawyers and staff in New York and Washington, and spend an additional $13m on citizen engagement, including protests and lobbying, a new front for an organisation that has primarily been a policy and legal group to date.

Sheryl Douglas, a receptionist at ACLU’s New York City headquarters since 1972, has been collecting some of the recent emails, letters and postcards.

“We commend your heroic efforts,” one said. “You give me hope,” said another.

Among the new donors was Andrew Mcdonald, 52, of Odessa, Missouri. “I’m ashamed to say I haven’t donated to any organisations in the past,” he said. “But things haven’t felt so threatening before either ... This time I felt like I couldn’t just sit here and do nothing.”

Another donor, Steve Berke, 35, of Miami Beach, Florida, said: “I think the ACLU is going to be a huge thorn in the side of the Trump administration. Trump has already demonstrated that he has a thin skin when it comes to anyone challenging his authority or power, but I’m confident that the ACLU will fight to protect American civil liberties.”

Over the years, the ACLU has been bitterly criticised for taking up unpopular causes, such as defending the rights of neo-Nazis or the Ku Klux Klan to demonstrate. Geraldine Engel, the ACLU’s deputy development director, said the recent outpouring had been heartening. “We were always unpopular, misunderstood,” she said.

The ACLU was founded in 1920 when a small group of idealists challenged the then attorney general Mitchell Palmer’s order that thousands of people labelled foreign anarchists or communists should be arrested without warrants. Many were deported.

The organisation was soon defending people’s constitutional rights to due process, privacy, freedom of assembly, speech and religion, and supporting minorities, including women, gay and transgender people, immigrants and prisoners.

Esha Bhandari, an ACLU attorney in New York, said the recent public reaction was encouraging to those who gave up bigger salaries to work for the nonprofit organisation.

“This is why we’re here,” the Columbia Law School graduate said. “The importance comes into sharp relief. We exist for moments like this. Lives are on the line.”