Whether its a race for the state legislature or for Congress, in most places most of the time around the U.S. — incumbents rarely lose. That makes special elections to fill unexpected vacancies, a rare opportunity to change the composition of legislative bodies.

There is one such special election going on to fill the unexpired term of the late Charlie Shannon for the Massachusetts State Senate, 2nd Middlesex District, that includes all or parts of Medford, Somerville, Winchester, and Woburn

There are four candidates currently in the home stretch of a hotly contested Democratic primary that will be held on August 30th. The candidates are Governor’s Councilor Michael Callahan of Medford; former state Rep. Joe Mackey of Somerville; state Rep. Paul Casey of Winchester; and Rep. Pat Jehlen (D-Somerville) — the apparent frontrunner and the favorite candidate of progressive reformers. She has racked up a remarkable list of endorsements including the Massachusetts Women’s Political Caucus, Massachusetts Nurses Association, Democracy for America; numerous education, environmental, labor, gay rights and women’s organizations; many current and former statehouse colleagues and former U.S. Secretary of Labor, Robert Reich.

“Pat Jehlen is the kind of principled, progressive Democrat we need in the State Senate,” said Reich. “She has spent her adult life fighting for education funding, better jobs, affordable healthcare, and a fair playing field for working families. She has been a leader on child labor issues, raising the minimum wage, and boosting local aid to help cities and towns.”

Jehlen is unequivocal on other key issues as well: “…I support equal marriage rights for gay and lesbian couples. I support a woman’s right to choose, and increased access to family planning options. And I oppose the death penalty in any form.”

She is an advocate for clean elections and reform in the Democratic Party, which she says “will be stronger if it embraces new activists — especially with such a groundswell of new progressive energy in recent years — and I support efforts to ensure those who are fighting for progressive values are welcome at the table.”

The winner of the Democratic primary is expected to face Republican Bill White, a Somerville alderman in the general election on Sept. 27th.

Jehlen’s latest endorsement comes from Progressive Democrats of Massachusetts, (PDM) — by the unanimous vote of the nine-member Statewide Coordinating Committee. (Full disclosure: I was one of the nine.) PDM is a statewide, grassroots organization that grew out of Robert Reich’s 2002 campaign for governor. It now has five chapters and a number of organizing committees at varying stages of development, and a statewide network of experienced activists dedicated to electing progressives to office. PDM was also active in a special election for state representative last spring.

“A four way primary election a few days before Labor Day means that voter turnout is almost certain to be low,” wrote PDM statewide chair Peter Dolan in an email to members. “A new State Senator could be chosen by a relative handful of Democratic primary voters.”

Its an aphorism of politics that endorsements, like lawn signs, don’t vote. So when PDM makes an endorsement it wants it to really mean something. Reflecting PDM’s strategy of targeting resources and volunteer help where they are most needed to take advantage of opportunities for progressive pick-ups in the statehouse, Dolan urged PDM members to contact the Jehlen campaign directly — or PDM — to participate in planned PDM-sponsored campaign activities.

Note to progressive reformers: Unexpected vacancies occur all the time. Expect the unexpected.