On a rain-soaked morning in late April, Mazie Hirono was walking from the U.S. Supreme Court back to her Senate office. She had just watched the nine Justices hear arguments on President Trump’s ban on immigrants from six countries with Muslim majorities and North Korea, and as she listened to arguments over the rights of immigrants and religious minorities, she couldn’t help but take the debate personally. Hirono, Hawaii’s junior Senator, is an immigrant from Japan and the chamber’s sole Buddhist. “Immigrants come here and leave everything that they know behind,” she says. “We have a sense of the opportunities that this country provides. We do not take those for granted.”

At age 70, Hirono has become one of the surprising avatars of what is known among liberals as the Resistance. She’s not the loudest voice in the Senate or its most polished speaker. But the first-term Senator has become one of the most outspoken critics of Trump’s behavior. “The President is very anti-immigrant. It’s a very xenophobic, nationalistic attitude,” she says. “Our country is made up of groups of immigrants who came here hoping for a better life. They created America. It’s a sad thing to have so many people not remember that, including Trump. His people came from another country, not to mention that his wife is an immigrant.”

These sharp rebukes have turned the soft-spoken Senator into a sudden star. “I’m one of the few members who calls him a liar. I don’t sugarcoat it and say he stretches the truth. No, the man lies every day,” she says. “To call the President a liar, that is not good. But it happens to be the truth.”

In January, when the President hosted a freewheeling, bipartisan meeting on immigration, Hirono confronted Trump directly. She was one of only two nonwhite faces at the table – the other was Senator Robert Menendez of New Jersey – and she prefaced her question with a preamble that established her credibility on the subject: “As the only immigrant serving in the United States Senate right now …” Hirono began. Early on in her tenure on Capitol Hill, officers sometimes stopped her from bypassing security lines, not recognizing her as a Senator because she didn’t look like most lawmakers. They quickly learned her often smiling face. “In our country, racism is never far below the surface,” Hirono says, sipping a midafternoon coffee. “I think the Trump campaign exposed the fault lines in our country.”

Late on election Night 2016, Hirono was at home in Hawaii, trying to decide what to say to supporters after Hillary Clinton’s surprise loss. Some of her advisers urged her to take a conciliatory tone, to pledge to work with the new President-elect, to give him the benefit of the doubt. That was the tack many of her colleagues would take in the numb days that followed. Not Hirono. “I didn’t feel like making a ‘Let’s give the man a chance’ speech,” she recalls. “His entire campaign was so negative and antithetical to everything I believe.”

For “a couple of months,” Hirono went into a self-imposed television blackout. She couldn’t handle what she saw as the President’s daily attacks on immigrants, women, democratic institutions and people who didn’t share his Christian faith. “There’s not a day that goes by that there isn’t a fresh assault on the body politic,” she says, leaning back in her chair in her seventh-floor corner office on Capitol Hill. “There’s hardly a day that goes by that my head doesn’t explode because, my goodness!”

A savvy legislator, Hirono spent 13 years in the Hawaii statehouse, eight years as the state’s lieutenant governor and six years in the U.S. House before winning her U.S. Senate race in 2012. At the Capitol, she kept her head down and focused on helping immigrants, veterans and the environment. She didn’t rush into battle just to hear the noise. But Trump has changed her approach to the office. Slowly, Hirono started saying in public what she was telling colleagues in private. Never one to run for the microphones or book TV appearances at all hours, she started saying yes to interview requests. Her elevated profile helped her avoid a once expected primary challenge.

Hirono has done all this while battling Stage 4 cancer, diagnosed in May 2017. In July she offered an emotional plea to protect President Obama’s health care law from a Republican-led appeal. “It’s hard for me to talk about this. I think you can tell. Give me a moment,” Hirono said in a speech from the Senate floor, delivered without prepared remarks. As her colleagues watched in silence, Hirono described being born at home in rural Japan, her sister’s death from pneumonia because the family didn’t have access to hospitals and a childhood spent living paycheck to paycheck. She then turned to the present. “I am fighting kidney cancer,” she said. “And I’m just so grateful that I had health insurance so that I could concentrate on the care that I needed rather than how the heck I was going to afford the care that was going to probably save my life.”

Some pundits suggested Hirono had “found her voice” or was “stepping out of the shadows,” both constructs the Senator finds grating and a tad sexist. After all, what man ever lacked a voice? “I had run other people’s campaigns. I had been doing political activities for a decade before I ever ran for office myself,” she says now. “That is so much the experience of women of my generation. We always feel as though we have to bring so much more to the table, and that never stops the guys.”

But Hirono is careful to ground her decisions in reality, including those about her health. “The first question I asked my doctor, when he told me of my diagnosis was, ‘Am I going to die anytime soon?’ He said no,” she remembers. “O.K., let’s talk about what kind of treatment I’m going to have,” was her reply. Doctors removed her right kidney and part of her seventh rib, where the cancer had spread. She is in ongoing immunotherapy treatment. She gets infusions every three weeks and says she expects to be in treatment for the long haul.

Hirono is running for a second term on this November’s ballot. She is popular enough in Hawaii that she hasn’t drawn a Democratic primary challenge, and the GOP does not fare well in the islands. Republicans in Washington are not planning to waste their money trying to boost a challenger. “I’m plugging away, not fading away,” Hirono says.

If Trump’s presidency has renewed her sense of purpose, it has not instilled a love for political combat. “I never refer to what I do as my career. What kind of career is it that you have to run for office every two years and go out there and ask total strangers to support you?” she asks. “It’s what I do. It’s my service.”

So why not retire?

“One person can make a difference,” Hirono says. “My mom changed my life by bringing me to this country.” Plus, she says, the President needs a counterbalance. “The battles that we win,” she says, “never stay won.”

This appears in the May 28, 2018 issue of TIME.

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Write to Philip Elliott at philip.elliott@time.com.