E-Mail This Article to a Friend SPECIAL ELECTION Four pols, $500,000, one winner BY DAVID S. BERNSTEIN

In a late-summer special-election primary, it’s hard enough to find anyone likely to vote, let alone to convince those people which of four viable options they should choose.

That’s why four Democratic candidates are spending about a half-million dollars to fill Charles E. Shannon’s vacant state Senate seat (Shannon died of cancer in April). Ten thousand voters throughout parts of Medford, Somerville, Woburn, and Winchester are expected to go to the polls on August 30.

Among the candidates, Patricia Jehlen, currently a state representative, has spent the most money — about $90,000 through the August 12 campaign-finance-reporting date — and has almost $60,000 more in the bank. Her long-standing support for progressive issues, including reproductive rights and gay marriage, has helped her gain the backing of some key special-interest groups, which have pitched in with their own mailings and provided her with volunteers.

Former state rep Joseph Mackey also has considerable liberal credentials. He’s spent just over $80,000; his big-ticket gamble was a flashy CD-ROM handout that not only pushed his message, but got him extra press coverage.

Due to a family medical emergency, Michael Callahan, a member of the Governor’s Council, has not filed his required pre-election finance report, but most of his spending can be gleaned from previous filings. Callahan claims to have spent roughly $80,000, and to have $33,000 left. He’s used mailings and automated "robo-calls," but also plans to knock on plenty of doors.

And finally Paul Casey, another representative looking to move to the Senate, has spent only a little over $40,000. That’s gone almost entirely to mailings and phone calls; he claims to have personally knocked on 10,000 doors, more than anyone in the race. As the least-liberal candidate — he opposes gay marriage, for instance — Casey may well enjoy the advantage of standing out against the others.

The winner will take on the lone Republican, William White Jr., in the general election.