Brookline attorney Shannon Liss-Riordan dropped out of the U.S. Senate race on Friday morning, leaving the Democratic primary on track to be a showdown between incumbent Sen. Edward Markey and U.S. Rep. Joseph Kennedy III.

"Unfortunately, sometimes some obstacles are too great to overcome, and that is the situation my candidacy for Senate finds itself in," Liss-Riordan said in a 7 a.m. statement. "In fairness to voters, to my family, and to the other candidates, I am ending my campaign for the U.S. Senate."

Liss Riordan, a labor lawyer who brought cases that forced policy changes at Starbucks, FedEx, Harvard University and American Airlines, had officially jumped into the race last May, launching a challenge against Markey three months before Kennedy confirmed speculation he was considering a run and opened a Senate campaign account.

Much of the attention on the race has focused on the battle between Markey, a longtime presence on Capitol Hill backed by dozens of state lawmakers, and Kennedy, who brings a famous family name to the contest and is pledging to be a fresh voice in the Senate. Another one-time Democratic contender, Steve Pemberton, ended his campaign last October, saying he had run into "an impenetrable wall of legacy and birthright."

As of Sept. 30, Liss-Riordan had $2.8 million in campaign cash, according to the Federal Election Commission, to Kennedy's nearly $4.3 million and Markey's almost $4.4 million.

In her announcement, Liss-Riordan did not throw her support behind either remaining candidate. She said she has enjoyed getting to know both Markey and Kennedy and that she takes comfort "in knowing that whoever wins this race will be a strong advocate for Democratic values."

"They are both inspiring people and I will be a proud constituent of either," she said.

Like both Markey and Kennedy, Liss-Riordan said she supports U.S. Sen. Elizabeth Warren's presidential bid. She said she will be in New Hampshire this weekend for Warren's "Women's Weekend of Action."

"While this campaign is over, my passion for fairness, for justice, and for equitable treatment continues to burn," Liss-Riordan said. "I've spent the last 30 years advocating for women, workers, and the disenfranchised, and I plan to spend the next 30 doing just that. We need more women in the Senate, and I hoped to add to that count this year. While I won't add to the number of women in Congress in 2020, I will be doing all I can over the next several months to help elect the first woman President, Elizabeth Warren."

May 5 is the deadline for candidates for Congress and statewide offices to submit their nomination papers to local officials. The state primary is scheduled for Sept. 1.

Markey, in a statement, wished Liss-Riordan well "as she continues her leadership in public service."

"The fact that Shannon participated in the primary's first debate on climate issues demonstrated that she knows we need to fight hard, not only to save the planet, but for justice and equity," he said. "We continue to need her voice and strength in this climate crisis movement and in Massachusetts politics."

Kennedy plans to be in Roxbury on Saturday to mark a campaign office opening in Nubian Square, formerly Dudley Square.