Hartford - Back in the spring, when state Rep. Chris Coutu, R-Norwich, was still pining for a seat in Congress, Sprague First Selectman Catherine Osten was a lead contender for the statehouse seat that he was giving up.

But she changed her goal for November after state Sen. Edith Prague of Columbia, a fellow Democrat, confided that she was retiring from the legislature and hoped that Osten would run instead for the Senate seat Prague had held since 1994.

"I'm not so happy about leaving my seat, to tell you the truth, but I have to be careful," Prague, the eldest member of the legislature at 86, said this week. Prague experienced a minor stroke in December. "I will feel better if somebody sits in my seat who will continue to do good for people in the district, and who's a strong labor person."

Osten, a former president of the supervisors' union for state prison guards, accepted Prague's invitation and endorsement for the 19th state Senate District, which includes Norwich, Ledyard, northeastern Montville, Franklin, Sprague, Lisbon, Marlborough, Hebron, Columbia and Lebanon.

But in agreeing to Prague's wishes, Osten entered a much tougher race. After Prague went public with her retirement plans, Coutu and state Rep. Tom Reynolds, D-Ledyard, announced their own candidacies for the Senate seat.

Coutu has defeated Osten once before, outpolling her 57 percent to 42 percent in 2010 when she tried to unseat him in the 47th Assembly District.

Reynolds - her opponent in the Aug. 14 primary - has racked up endorsements by union and political leaders all summer since edging Osten for the Democratic nomination at the party's convention in May.

But neither man possesses the Prague endorsement. Next month's primary will test whether that ace card translates to votes.

"To be endorsed by Senator Prague is the best endorsement any candidate can ever have," Osten said in an interview.

She doesn't regret accepting Prague's challenge to drop her promising bid for the House and go for the Senate because, she said, she really believes she can win.

"That's exactly why people should choose me," Osten said. "I knew this was going to be a hard-fought battle for a position where I could make a significant difference for the people in this district and the state."

Osten, elected first selectman in 2007, received her biggest union endorsement to date Tuesday from AFSCME Council 4. She previously had been endorsed by CSEA/SEIU 2001, the umbrella union for state correction supervisors.

But Reynolds gained endorsements from both statewide teachers' unions, the state machinists council, Backus Federation of Nurses, the Norwich-New London Building and Construction Trades Council, and the United Automobile, Aerospace and Agricultural Implement Workers of America Region 9A.

He also holds endorsements from elected Democrats in all of the district's towns and from 12 of the region's 13 state legislators (all but Prague).

Reynolds this month became the first of the three to qualify for public financing through the Citizens' Election Program. To be eligible, a candidate must raise a minimum of $15,000 in contributions of between $5 and $100, and at least 300 contributors must reside in the district.

"I obviously couldn't be happier with the response," said Reynolds, a four-term state representative and deputy majority leader. "I have 18 years of experience in local and state governments that far exceeds either of my opponents."

He later added: "Coutu has never gotten a bill passed, so I am the only candidate with an actual legislative record of delivering priorities for the district."

Coutu, the endorsed Republican, has been busy collecting the roughly 400 signatures needed for a potential endorsement and second line on the ballot with the Independent Party of Connecticut.

Both Reynolds and Osten are scheduled to be in Hartford today seeking the endorsement of the Connecticut Working Families Party, and Coutu said he doesn't want to be at a disadvantage in November if one of them snags that second ballot line.

"If you look at the people they endorse, it's 99 percent Democrats," Coutu said. "The Working Families Party never endorses working families or military families like mine."

Osten received 243 of her 3,268 votes in 2010 under the Working Families line but still lost to Coutu by 1,094.

"I'm trying hard to bring balance to Hartford by winning a very important seat," Coutu said. "I am the only elected official in all southeast Connecticut who's not in the majority, so I will question taxes and question spending like the busway in Hartford, and you're not getting that from the other representatives."

Lindsay Farrell, executive director of the Connecticut Working Families Party, said party leaders also would have met with Coutu had he sought their endorsement. However, she pointed out how he voted against the party's big issue last year - mandated paid sick days for service industry employees in companies with 50 or more workers.

"Unions represent a lot of hardworking people," Farrell said.

Osten has been knocking on doors across the district and calling prospective primary voters. She is highlighting her efforts to retain jobs at Fusion Paperboard and Atlantic City Linen Supply Inc., two local firms facing water and sewer rate increases in Norwich.

She noted how Prague lost the Democratic convention endorsement in 1994 but through a lot of hard work over the summer went on to beat the longtime incumbent in the primary, Kenneth Przybysz, and win the seat that November in a costly three-way battle.

"I plan on fighting like Edith did," Osten said. "Edith door-knocked until her hands bled. I told her it won't be my hands that bleed, but my feet will be blistered from stem to stern."

j.reindl@theday.com