Lawyers for the American Civil Liberties Union are gearing up for what is expected to be a crucial showdown in the US supreme court in 2018 over Donald Trump’s Muslim travel ban, as they enter the second year of an epic battle against the president’s populist – and frequently arguably unconstitutional – agenda.

The nation’s oldest and largest civil liberties group has found itself on the frontline of legal resistance to an executive branch that is proving to be historically hostile towards constitutional rights. Since Trump took power on 20 January, the organization has launched 113 legal actions attempting to block his extreme rightwing ambitions.

Those actions included an unprecedented 57 lawsuits brought against the most egregious aspects of the Trump project. Having warned Trump before he entered the White House that he would have to “contend with the full firepower of the ACLU at your every step”, the organization has lived up to its word.

It has successfully challenged the administration in its efforts to deport 11 million undocumented immigrants, cut off access to abortion, ban transgender people serving in the military and overturn Obamacare.

“Donald Trump is certainly the most dangerous president we’ve had in my lifetime, and possibly ever,” said David Cole, the ACLU’s national legal director, who leads a central team of 100 lawyers. “But he’s also the most frustrated president in my lifetime, maybe ever.”

Arguably the ACLU’s most significant action was to challenge his initial Muslim ban, issued within his first week in the White House, that barred Syrian refugees and visitors from seven majority-Muslim countries from entering the country. The ensuing legal tussle between the ACLU and the president has been drawn out over months and through many iterations and is currently before the US fourth circuit court of appeals where a ruling is expected imminently.

Cole said he was confident that the court would strike down the Muslim travel ban as unconstitutional. Last week in a parallel case initiated by the state of Hawaii, the US ninth circuit court of appeals once again found that Trump had exceeded his authority. But it won’t end there. The Trump administration is almost certain to take one or other of the cases up to the US supreme court, which in turn is likely to give its final verdict by the end of its term in June.

For the ACLU’s legal director it will be the ultimate test of what he believes is Trump’s “unique disregard and in some instances contempt for basic constitutional norms”. It will also be a test of the limits of the president’s power, which so far has turned out to be surprisingly limited – thanks in no small part to the ACLU’s dogged opposition.

The president has the advantage of conservative control of both chambers of Congress, two-thirds of state legislatures and, since he appointed Neil Gorsuch to the highest court, the US supreme court. Yet the $1.5tn tax cuts that were rushed through Congress this month were the president’s first and only major legislative achievement.

Colin Kaepernick at the ACLU’s annual bill of rights dinner earlier this month. Photograph: Buchan/Variety/Rex/Shutterstock

“We have managed to limit the damage that Trump has wrought. And we’ve helped to mobilize the spirit of liberty in citizens that in the long term will encourage them to be engaged in defense of those values,” Cole said in an interview with the Guardian.

The 113 legal actions this year span the range of Trump’s attacks on civil liberties, from his attempts to turn the screws on undocumented immigrants, suppress voting through its Orwellian-named “Commission on Election Integrity”, unpick equal employment rights for women and the LGBT community, and increase surveillance on ordinary Americans. In addition to the lawsuits, the ACLU has issued 69 freedom of information requests and several ethics complaints including one against the attorney general, Jeff Sessions, for his misleading comments to a Senate committee over communications with Russia.

That’s an impressive record for an organization that is utterly outgunned by the federal government when it comes to legal firepower. The ACLU’s national team of 100 lawyers under Cole is pitted against 19,000 attorneys employed by just six key federal departments.

Trump has also inspired citizens in the US to engage in civil rights and liberties. Apathy right now is not our problem David Cole

But that daunting disparity is leveled out to some degree by the 200 additional lawyers deployed by the ACLU through its 53 affiliates in each of the states, and there are plans to bring a further 100 staff on board to meet the Trump challenge. On top of that, the organization has been propelled by the rocket fuel of its membership which has exploded from 425,000 before the presidential election to 1.6 million today.

Cole, who is on leave from Georgetown University where he is a constitutional law scholar, is big on harnessing the potential of members. Last year he published Engines of Liberty, a book exploring the phenomenon of citizen activism, and in the wake of Trump he and the ACLU have launched a new initiative, People Power, that aims to encourage peaceful protest and engagement.

“When you see the Women’s March, the airport demonstrations, the town halls around Obamacare, the recent elections in Virginia and Roy Moore’s defeat in Alabama – these are all examples of citizen mobilization inspired by the Trump threat,” Cole said.

One of the sharpest examples of “people power” was the eruption of public anger at Trump’s initial Muslim ban. The ACLU was already primed for action, having drawn up before the inauguration a seven-point plan to take on Trump, and within hours of the travel ban executive order being issued they had filed suit.

Hours after that mass protests erupted at US airports as protesters turned out in their thousands to oppose what they saw as an attack on American values. It was a vivid manifestation, Cole believes, of “people power”.

“That said to me that we are in a different climate today in terms of citizen engagement. This was a measure targeted at foreign nationals – not at Americans at all – and yet tens of thousands of Americans turned out to protest at the airports. They knew that it would be their own rights on the block next time.”

Cole is cautious in predicting the outcome of a supreme court ruling on the Muslim ban. On the one hand he’s clear that the president has a strong hand in terms of his executive power over immigration and national security.

On the other hand, as a candidate Trump called for the “total and complete shutdown” of Muslim entry to the US. “He could not be more explicit about what he was doing – effectively putting up a ‘Muslims Keep Out’ sign. The conservatives on the supreme court are very sensitive about religious freedoms, and you can’t get a more direct violation of the constitution than that,” Cole said.

With so much still hanging in the balance, Cole had no illusions about premature victory. “There’s plenty of damage that Trump can do and that he has already been doing.”

Above all, the Trump administration has shown itself to be adept at appointing relatively young and highly conservative judges to federal benches. “This is about the long term,” Cole said. “These people are not going to be there for four years but for their lifetimes, maybe 30 or 40 years. That’s very concerning.”

Against that, he draws hope from the outpouring of support the ACLU has received from ordinary Americans. “I don’t think we’ll win every battle and Trump will continue to do tremendous damage. But the silver lining is that he has also inspired citizens in the United States to engage in civil rights and liberties. Apathy right now is not our problem.”