Sen. Kirsten Gillibrand, D-New York, is expected to announce her plans to form a presidential exploratory committee on Tuesday's "Late Show with Stephen Colbert," according to a person familiar with her planning.

Gillibrand is scheduled to appear on the CBS late night program on Tuesday night. If she announces her exploratory committee, as expected, she would become the second U.S. senator — and second female senator — to announce plans to begin the legal process of running for the Democratic presidential nomination in 2020. The 52-year old is one of the most active and outspoken critics of President Trump and she earned acclaim from party activists when she began voting against all of the president's cabinet and senior government nominees in early 2017.

Gillibrand would join a Democratic presidential field beginning to take shape. Over the weekend, former Housing and Urban Development (HUD) secretary Julián Castro formally launched his presidential bid, while Sen. Elizabeth Warren, D-Massachusetts, launched an exploratory committee on New Year's Eve. Rep. Tulsi Gabbard, D-Hawaii, said over the weekend that she plans to form an exploratory committee in the coming days. Several other senators, current and former governors and mayors, and former Vice President Joe Biden are still mulling potential campaigns.

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Spokespeople for Gillibrand declined to comment. In recent days, she has announced that her Senate chief of staff, Jess Fassler, and Meredith Kelly, a former communications director for the Democratic Congressional Campaign Committee (DCCC), would be joining the senior staff of her prospective campaign.

Sen. Kirsten Gillibrand

Gillibrand is the junior senator from the state of New York and was appointed to the seat in January 2009 to replace former Sen. Hillary Clinton, who had stepped down to serve as secretary of state. Previously, she was known as a moderate Democrat representing New York's North Country who openly touted her support of gun rights. But her positions on many policy issues took a decidedly liberal turn once she began serving the Empire State and she became an outspoken proponent of stricter gun laws, comprehensive immigration reform and women's rights. She won her first full term to the Senate in 2012 and was reelected this past November.

In the Senate, she's used her perch on the Armed Services Committee to push for the repeal of the "don't ask, don't tell" policy that banned gay men and lesbians from serving in the military. Last year she was the first Senate Democrat to call on former Sen. Al Franken, D-Minnesota, to resign amid accusations of inappropriate behavior towards women. She also sits on Senate committees focused on agriculture, aging and the environment.

Before running for Congress in 2006, Gillibrand once served as special counsel to then-HUD Secretary Andrew Cuomo and later as a Manhattan-based attorney who worked for a firm that represented Philip Morris, the cigarette company.

Gillibrand graduated from Dartmouth College and received her law degree from University of California Los Angeles. She is the author of the book "Off the Sidelines: Speak Up, Be Fearless, and Change Your World," and a children's book named "Bold & Brave: Ten Heroes Who Won Women the Right to Vote."