Jessica Scarane, a teacher, was defeated by more than 50,000 votes.
Coons is a close ally of Democratic presidential nominee Joe Biden, who voted in person this week.
On Sept. 15, the senator wrote on Facebook: “Serving the people of Delaware in the United States Senate is the honor of a lifetime, and I’m humbled by the support our campaign received from Delaware Democrats up and down our state in this primary election.
“My primary opponent, Jessica Scarane, ran a campaign focused on critical issues such as strengthening healthcare, raising wages, and combating climate change. While we have different approaches, we share the same goals of building a more just, sustainable, and inclusive Delaware.”
Scarene said in a concession statement that she was grateful to family, friends, and supporters.
“We didn’t win tonight, but this was never just about one election. We launched this campaign with three main goals: prioritize individual voter contact, particularly with folks who feel politically disenfranchised, train up a new group of local organizers, and focus on a core message that every person deserves healthcare, housing, and living wages,” she said.
“From the successes of progressive down-ballot candidates, it’s clear that the popular mandate exists for this platform. I’m so proud of the work that we’ve accomplished and am excited to keep fighting alongside all of you for the state and country we deserve.”
Coons said he’d be working “hand in hand” with Biden to win the election.
Coons is part of a growing cohort to survive challenges from younger, further-left candidates.
Rep. Eliot Engel (D-N.Y.) has remained the biggest Democrat to be defeated in a primary so far this cycle.
Political activist Lauren Witzke won the Republican primary over attorney James DeMartino and will face the incumbent.
According to Witzke’s campaign website, she is running on immigration reform, restoration of the family unit, and against abortion, among other issues.
Democrat Gov. John Carney, meanwhile, won his primary with 85 percent of the vote.
Carney will face Julianne Murray, a Republican who emerged from a contested primary race.
Murray has taken aim at Carney’s treatment of businesses, vowing to treat small companies the same as big box stores if she’s elected.