U.S. Sen. Edward Markey has faced just a handful of primary challenges in his more than four decades in Congress, but the Malden Democrat was served notice Monday that his 2020 re-election bid will not go uncontested.

Shannon Liss-Riordan, a prominent Brookline labor attorney, announced her plans to run against Markey in next year's Democratic primary, pitching herself as a "fresh voice" who could shake up the status quo during a time of partisan gridlock.

Sixteen months before the election, she jumped into a pool of Democratic politics still churned up by Congresswoman Ayanna Pressley's upset defeat of popular incumbent Michael Capuano last September. And while running statewide will present a different set of challenges than Pressley faced when taking on an incumbent, Pressley's victory -- as well as John Tierney's loss to U.S. Rep. Seth Moulton in 2014 -- showed that the power of incumbency has its limits in Massachusetts.

"I have been fighting for workers for 20 years and I feel like the voices of working people need to be heard more in Washington today," Liss-Riordan said. "The reason I'm doing this campaign is I think I can bring a new perspective. We need more women in the Senate, in particular."

Liss-Riordan, in an interview, didn't take on Markey directly over policy, but she described herself as "incredibly energized" by watching Pressley and U.S. Rep. Alexandria Ocasio-Cortez of New York topple incumbent Democrats last year.

"They showed that coming in with passion and beliefs, talking about what they believe in, outsiders can win against long-time incumbents. Women can win and I'm hoping to capitalize on the same energy," Liss-Riordan said. She said she's worried about corporations "profitting on the back of working people," and the "assault" by Republicans on women's reproductive rights.

Read more: Labor Attorney To Challenge Ed Markey For US Senate Seat

As someone who has served in Congress since the 1970s, Markey may be vulnerable to charges of becoming a creature of the establishment. Supporters, however, will note that the Malden Democrat has also become one of the leading voices in the Senate on progressive issues like climate change.

Working with Ocasio-Cortez, Markey this year introduced the Green New Deal. He said Monday he won't let the fact that he has a Democratic primary challenger change the things he cares about, from net neutrality and a woman's right to choose to a welcoming immigration policy and stronger gun control.

"Those are the issues I've been fighting for throughout my entire career, and I'm going to continue to fight for them," Markey told reporters after addressing a New England Council luncheon. "I'm as energized as I have ever been because Donald Trump every single day is launching assaults on all of the issues that are at the core of the identity of the state of Massachusetts and our entire country."

Markey, in a speech, said that under Republican leadership the Senate has increasingly become a "graveyard" for those policies he has championed, but he did predict action on a couple of key issues, including a bill he has worked on with Sen. John Thune to "abolish robocalls."

"There are no blue robocall or red robocalls," he said.

He also said he was "quite confident" the Senate would take up a comprehensive bill for online privacy for children this year, and said he had hopes for a $300 billion surface infrastructure bill that would send $5 billion to Massachusetts over five years.

On climate change, Markey said the science is clear that the Earth is "dangerously warming," and said the United States must take the lead on developing new technology and solutions that will create hundreds of thousands of new clean energy jobs.

"Ultimately if Donald Trump gets a second term, it's the equivalent of a death sentence for the planet," Markey said.

One early sliver of daylight between Markey and Liss-Riordan is on the issue of impeachment.

Markey said Monday that he wants special counsel Robert Mueller and other key witnesses to testify before Congress first. That stands in contrast to Liss-Riordan, who said President Donald Trump should be removed from office immediately.

"We need first to establish, through an investigation, what exactly happened before we reach the question of impeachment. That is, in fact, the correct way to proceed," Markey said.

Liss-Riordan said she "absolutely" supports impeachment.

"Donald Trump is an embarrassment to our country," she said. "He's obstructed justice. He's stonewalling investigations. He's an embarrassment to us around the world and he's unfit for office."

In the newest Morning Consult poll, Markey's approval rating was 51 percent, compared to 22 percent who disapproved of the job he's doing. That was good enough to make him the 16th most popular member of the Senate.

Markey has served in the Senate since 2013 when he prevailed over fellow Congressman Stephen Lynch in a special Democratic primary with 57 percent of the vote, and went on to defeat Republican Gabriel Gomez 55 percent to 45 percent.

He also put his name on the ballot 19 times between 1976 and 2013 running for the U.S. House, participating in just four contested primaries, including his initial victory in a 12-way race race for an open House seat and a five-way primary in 1984. He also beat back challenges in 1980 and 2002, winning 85 percent of the vote both times.

Liss-Riordan, however, may be a little more well-known than Markey's past challengers.

She has successfully sued corporate giants like Uber and Starbucks on behalf of workers, and after winning a case against American Airlines securing $325,000 in lost tips for baggage porters at Logan Airport, one of the plaintiffs dubbed her "Sledgehammer Shannon," according to Mother Jones.

In both her campaign video and in an interview, Liss-Riordan mentioned the Brett Kavanaugh Supreme Court confirmation hearings as a moment that helped solidify her desire to run.

"I was horrified by Brett Kavanaugh getting sworn in and had flashbacks to the horror of the Anita Hill hearings," she said. "I was trying to explain that to my 13-year-old daughter and I didn't know how to explain it."

A Harvard Law School graduate, Liss-Riordan said she plans to step away from the firm Lichten & Liss-Riordan that she co-founded in 2009 to campaign full time.

Markey raised $3.6 million in the first quarter of 2019, and reported having $3.5 million in cash-on-hand at the end of the fundraising period. Liss-Riordan opened her committee in April, and has not yet had to file a report.

"It's an unfortunate side of politics that money has come to pay such a role," Liss-Riordan said. "I'm going to do what I need to do to get the message out there, but my focus will be on meeting with regular people and asking them to give at whatever level they can."

Liss-Riordan is still staffing up her campaign, according to a spokesman, while Markey on Monday announced a number of hires for his re-elect campaign. The senator's state finance team will be led by Colleen Coffey and Michael Pratt, and his field organizing effort will be run by Carl Nilsson, who was Markey's 2013 field director and President Barack Obama's 2012 state director.

The treasurer of Liss-Riordan's campaign committee is listed on campaign finance documents as Paul Brountas, Jr., the son of a former trusted advisor to Michael Dukakis.

Both Markey and Liss-Riordan are backing Massachusetts Sen. Elizabeth Warren for president in 2020, and Liss-Riordan said she supports both Warren's proposed "ultra millionaires tax" and the state-level "millionaires tax" moving toward the 2022 ballot.

"I'm all in for Elizabeth Warren. I think she's terrific. I think she's incredibly smart," she said.