Sen. Kirsten Gillibrand (D-N.Y.), seen in July, announced she will run for president. (Evelyn Hockstein/For The Washington Post)

Sen. Kirsten Gillibrand of New York announced Tuesday that she will run for the Democratic nomination for president in 2020, in a campaign that is expected to lean heavily on gender issues and imagery.

She told host Stephen Colbert on CBS’s “Late Show” that she believes she has “the compassion, the courage and the fearless determination” necessary.

“The first thing I would do is restore what’s been lost: the integrity and the compassion in this country,” she said. “I would bring people together to start getting things done.”

Gillibrand, 52, is most well known for her efforts to combat sexual assault in the military and on college campuses, to repeal the military’s policy of “don’t ask, don’t tell” and to make it easier for Capitol Hill staffers who have been sexually harassed or assaulted to report their experiences.

The senator has latched on to the burst of activism prompted by President Trump’s election and his policies, a movement that’s largely driven by women. She called the 2017 Women’s March on Washington “truly the most inspiring moment of my entire life” and joined the protesters who challenged Brett M. Kavanaugh’s nomination to the Supreme Court last fall. She also stood up to fellow Democrats as the #MeToo era dawned, criticizing then-Sen. Al Franken of Minnesota and former president Bill Clinton for their alleged inappropriate behavior toward women.

Gillibrand is also a vocal critic of Trump, and she has voted against his political appointees and positions at a higher rate than most Democrats. The president responded in December 2017 by attacking her in a tweet that she called “a sexist smear.”

With the announcement made, Gillibrand plans to spend time with her husband and two sons on Wednesday in Troy, N.Y., where she lives and where her campaign will be headquartered. On Friday, she will start a three-day tour of Iowa. Gillibrand emphasized her family in Tuesday’s announcement.

“I’m going to run for president of the United States because, as a young mom, I’m going to fight for other people’s kids as hard as I would fight for my own. Which is why I believe that health care should be a right and not a privilege,” she said. “It’s why I believe we should have better public schools for our kids because it shouldn’t matter what block you grow up on. And I believe that anybody who wants to work hard enough should be able to get whatever job training they need to earn their way into the middle class.”

Since Gillibrand was appointed to the Senate in January 2009 to fill the seat left open when Hillary Clinton became secretary of state, she has undergone a rapid and dramatic political shift, abandoning many of the centrist positions she held during her time as a congresswoman from Upstate New York and becoming one of the Senate’s most liberal members.Trump’s campaign manager Brad Parscale has seized on this transformation, pointing to Gillibrand as an example of Democratic “political contortionism” — even as Trump, too, has shifted in nearly all of his policy positions.

Gillibrand has said that she developed a passion for politics while growing up in Albany. Her maternal grandmother was an influential political organizer, and her mother worked as a lawyer, had a black belt in karate and shot the family’s Thanksgiving turkey each year.

Gillibrand studied at Dartmouth College and the University of California at Los Angeles School of Law, then worked as a corporate attorney in Manhattan for more than a decade. She helped represent the tobacco company Philip Morris in the 1990s amid a federal investigation — controversial work, at least among Democrats, that she has struggled to defend. In the late 1990s, Gillibrand volunteered on Clinton’s first Senate campaign and distinguished herself as an aggressive fundraiser, a skill that has been key to her political career.

“In my adult life, politically, no one has inspired me to get off the sidelines and truly make a difference more than Hillary Clinton has,” Gillibrand wrote in a January 2016 essay endorsing Clinton for president. Clinton wrote a foreword for Gillibrand’s 2014 memoir.

Gillibrand first ran for office in 2006, beating a four-term Republican in a conservative congressional district that includes the Albany suburbs. In the House, Gillibrand joined the Blue Dog Democrats, a centrist group, and embraced many conservative positions. Her support of gun rights legislation earned her a 100 percent approval rating from the National Rifle Association. She opposed amnesty for undocumented immigrants and voted to cut off some federal funding to New York City until the city cracked down on illegal immigration.She opposed legalizing same-sex marriage.

When Clinton resigned her Senate seat in January 2009, New York Gov. David Paterson (D) appointed Gillibrand, who was then barely known outside Upstate New York — angering many Democrats who considered Gillibrand too conservative.On the Hill, members of the New York delegation nicknamed Gillibrand “Tracy Flick” after the bubbly, blond and ambitious character played by Reese Witherspoon in the movie “Election.”

Some of her policy positions rapidly changed. The night before her appointment was announced, she called a gay rights group to profess her full support for same-sex marriage. As she voted for gun-control measures, her NRA rating fell to an F.

Gillibrand said in a CBS News interview last year that as she expanded her views beyond “the lens of Upstate New York,” she realized that her gun rights and immigration positions were “wrong.”

“I just didn’t take the time to understand why these issues mattered because it wasn’t right in front of me. And that was my fault,” Gillibrand said in the interview.“It was something that I’m embarrassed about and I’m ashamed of.”

She won a special election in 2010 with 63 percent of the vote and followed with 72 percent of the vote in 2012, when she earned her first full term, and 67 percent in November. Her last campaign came as Gillibrand navigated intraparty divisions over how to handle the #MeToo movement.

In November 2017, Gillibrand said that Bill Clinton should have resigned during his presidency following his affair with a White House intern. That angered some Clinton loyalists. Former adviser Philippe Reines, who tweeted: “Over 20 yrs you took the Clintons’ endorsements, money, and seat. Hypocrite. Interesting strategy for 2020 primaries. Best of luck.”Three weeks later, Gillibrand called on her colleague Franken to resign following accusations of sexual misconduct from several women. She was the first prominent Democrat to do so, and many others followed, although she also faced criticism from members of her party and some major donors .

“Enough is enough,” Gillibrand wrote in a Facebook post. “The women who have come forward are brave and I believe them. While it’s true that his behavior is not the same as the criminal conduct alleged against Roy Moore, or Harvey Weinstein, or President Trump, it is still unquestionably wrong, and should not be tolerated.”A few days later, Gillibrand called on Trump to immediately resign because he had been accused of sexual assault by more than a dozen women.

Trump responded the next day in a tweet: “Lightweight Senator Kirsten Gillibrand. . . someone who would come to my office ‘begging’ for campaign contributions not so long ago (and would do anything for them), is now in the ring fighting against Trump.”

Gillibrand said the tweet was “a sexist smear intended to silence me.”

One of the most prolific fundraisers in Congress, Gillibrand has raised more than $56 million during her political career, including $20 million between 2013 and 2018. But her prowess has led to criticism that she was too cozy with Wall Street.In 2013, the Daily Show’s John Oliver confronted Gillibrand about campaign donations from Wall Street and said: “What I deeply want to know is: What do you have to do for that? What is required of you for that money? Because it makes me uncomfortable.”

Gillibrand responded that it was her job to represent New York and its people, which includes those employed on Wall Street.She noted that she has called for more regulation of the banking industry and voted against the government bailout of banks. Nearly a year ago, Gillibrand stopped taking money from corporate political action committees, following the example of Sens. Bernie Sanders (I-Vt.) and Elizabeth Warren (D-Mass.).

During her reelection campaign last year, Gillibrand promised that she would “serve my six-year term” and not challenge Trump in 2020. Her Republican opponent replied: “Honestly, I don’t believe that.”

Two days after she was reelected, Gillibrand said in a late-night television interview that she was considering a run.

“I believe it is a moral question for me,” she said. “I’ve seen the hatred and the division that President Trump has put out into our country, and it has called me to fight as hard as I possibly can to restore the moral compass of this country.”

Philip Bump contributed to this report.