HELENA, Mont. — Montana’s Democrats, scrambling to salvage their political fortunes after plagiarism charges forced Senator John Walsh to end his election bid, chose a high school math teacher and one-term state legislator on Saturday as their nominee for a fiercely contested Senate seat.

The nominee, Amanda Curtis, 34, who grew up in a family stalked by poverty and tragedy, cast the stakes of the election in stark economic terms, saying it was about “millionaires versus the middle class.” As the daughter of a union worker whose family sometimes relied on food stamps to buy groceries, Ms. Curtis said she knew what it meant to worry about bills and the price of gas.

“America is breaking its promises,” Ms. Curtis told delegates who gathered for a special nominating convention here at the county fairgrounds. “This is the worst job market in a generation, but the stock market’s doing just fine. Wall Street’s doing great. This recovery has not reached the rest of us.”

She also acknowledged the headwinds facing Democrats in Montana, saying the party had been “outspent, outgunned” and left for dead after Mr. Walsh’s candidacy was upended by a report in The New York Times that he had plagiarized large portions of his thesis at the Army War College in 2007. Mr. Walsh was appointed to the Senate in February to finish the term of the retiring Democrat Max Baucus, and Democrats had hoped his military bearing and time in Iraq could help them fend off a well-financed challenge to a seat they have held for a century. Instead, Mr. Walsh dropped out of the race on Aug. 7.