After months of behind-the-scenes work, former Hartford mayor Luke Bronin pulled a stunning upset Monday night by defeating longtime U.S. Rep. John B. Larson at the Congressional convention — setting up a potentially bitter primary in August that pits hardscrabble East Hartford against upscale Greenwich.

Bronin, 46, repeatedly said during his campaign that he would bring more energy than the 77-year-old Larson and would battle every day in the fight against President Donald Trump’s Republican administration. A former U.S. Navy Reserve intelligence officer in Afghanistan who grew up in Greenwich, Bronin is known for an intense work ethic and for outworking his opponents in previous elections.

He delivered a brief, rousing speech to the delegates at Goodwin University in East Hartford as Larson stood silently in the front row about 40 feet away. The announced tally was 51.2% for Bronin and 48.8% for Larson.

“We just saw a political earthquake in this convention,” Bronin said as his supporters cheered.

A skilled politician, Bronin worked closely with the delegates and captured the votes in multiple towns. A huge total of 50 delegates from his hometown of Hartford proved to be a major boost for Bronin.

“This is one stage in the process, but it’s an important one,” Bronin told reporters after his speech. “It sends a powerful signal to Democrats out there that they have a real choice. What we usually see at these conventions is a coronation of the incumbent. What we saw tonight was a sea change. What we saw tonight was the most active Democrats in the district sending a message that we want change.”

Asked by The Courant when he knew he had enough support, Bronin said, “I didn’t know I had it until they read the numbers. But look, coming into this convention, I knew we had a lot of strength.”

State Rep. Jillian Gilchrest of West Hartford watches as votes are counted at the convention on May 11 in East Hartford. She is standing with U.S. Rep. John Larson's campaign manager, Greg Gerratana. Gilchrest later qualified for the primary by reaching the minimum of 15% of the delegates. (Christopher Keating/Hartford Courant)State Rep. Jillian Gilchrest of West Hartford watches as votes are counted at the convention on May 11 in East Hartford. She is standing with U.S. Rep. John Larson’s campaign manager, Greg Gerratana. Gilchrest later qualified for the primary by reaching the minimum of 15% of the delegates. (Christopher Keating/Hartford Courant)

In a key development, state Rep. Jillian Gilchrest of West Hartford qualified for the three-way primary with the minimum of 63 votes, representing 15% of the delegates. She was congratulated by Larson after the announcement at Goodwin University in East Hartford. Neither Bronin nor Larson would explain exactly how Gilchrest went from having 8.8 percent of the votes on the first round and then seeing a jump in her total to exactly 63 delegates – or 15% – after the delegate “switches” allowed them to change their votes.

“In a democracy, the more, the merrier,” Larson said, adding that he had hoped that Hartford attorney Ruth Fortune would qualify for the primary. Fortune received 1.9% and told The Courant after the convention that she fully intends to raise the necessary 3,734 signatures by June 9 to gain a spot on the ballot.

Since neither Bronin nor Larson achieved 50% on the first round, the delegates were required to vote again in a second round.

Larson’s loss was stunning to his supporters, who thought he had as many as 220 delegates that would have guaranteed a first-ballot victory with more than 50% of the delegates. But that did not happen.

Despite the setback, Larson vowed to deliver a spirited campaign for the August 11 primary.

“This is democracy at work,” Larson told reporters. “But this is the first step.”

Mayberry vs. Greenwich

Larson is already framing the battle as a fighter with a background in public schools and public housing versus an Ivy League graduate and Rhodes Scholar who grew up in Greenwich – one of the richest towns in the nation.

“Now I think it’s going to be very clear : it’s Mayberry versus Greenwich,” Larson said. “We’re in this fight on behalf of the working people that I have served for 28 years in the United States Congress. … My fight is going on now. It doesn’t wait until November. … Project kids don’t give up. Project kids just lean into a fight like this. I’m looking forward to it.”

Larson’s supporters had been confident for months, saying that Larson had the experience and a long history of winning elections, including 14 consecutive elections dating back to 1998 in his first run for Congress. He had a long series of wins for state Senate before running for governor and losing in a primary to liberal Democrat Bill Curry in 1994. He then made a political comeback and has served in Congress alongside the top leaders through the years, including former House Speaker Nancy Pelosi and current Democratic leader Hakeem Jeffries of Brooklyn, N.Y.

This story will be updated.

U.S. Rep. John Larson talks to a supporter at the convention Monday, May 11. (Christopher Keating/Hartford Courant)U.S. Rep. John Larson talks to a supporter at the convention Monday, May 11. (Christopher Keating/Hartford Courant)