Massachusetts Senator Elizabeth Warren Elizabeth WarrenNo new taxes for the ultra rich — fix bad tax policy instead Democrats back away from quick reversal of Trump tax cuts It's time for newspapers to stop endorsing presidential candidates MORE reveals why she decided against running for president in 2016 in a new book to be released Tuesday.

In "This Fight Is Our Fight," the progressive favorite recalls asking her husband, Bruce Mann, for his thoughts on a 2016 run for the White House.

While her husband was supportive, he was wary that a presidential run would be more intense than her 2012 Senate race against then-incumbent Sen. Scott Brown.

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"The Senate thing was bad enough, and running for president would be worse — a lot worse,” he had warned her.

During the 2012 Senate campaign, Brown often referred to Warren as "Professor Warren," a shot at her Harvard credentials, and targeted her for claiming Native American ancestry during her hiring process. Despite the attacks, Warren won the election by 8 points.

Warren was floated by the more liberal wing of the Democratic Party as an alternative to Hillary Clinton Hillary Diane Rodham ClintonBarr criticizes DOJ in speech declaring all agency power 'is invested in the attorney general' Virginia Democrat blasts Trump's 'appalling' remark about COVID-19 deaths in 'blue states' The Hill's Campaign Report: Biden asks if public can trust vaccine from Trump ahead of Election Day | Oklahoma health officials raised red flags before Trump rally MORE last year. In the end, Warren declined to run, opening the door for Vermont Sen. Bernie Sanders Bernie SandersSenate Republicans signal openness to working with Biden Hillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll Schumer, Sanders call for Senate panel to address election security MORE to challenge Clinton for the Democratic nomination.

Many political observers now see Warren as a likely presidential candidate in 2020, when Democrats will be looking for a new standard-bearer to take on President Trump.

Despite Warren's refusal to run in 2016, she remained a target for Republicans during the election. During rallies, then-candidate Trump referred to her as "Pocahontas" as a way to make fun of her claiming Cherokee heritage.

In her book, Warren talks about how the Clinton and Sanders campaigns both pressured her for an endorsement.

"I didn’t want to undermine either of our candidates or to short-circuit any part of that debate,” Warren writes.