Two high-profile congressional primaries in Massachusetts this week yielded very different results:  a stunning upset and a race too close to call.

The biggest surprise of the night was touted nationally as a harbinger of change in the Democratic Party.  In the Third Congressional District, Boston City Councilor Ayanna Pressley last night soundly defeated U.S. Rep. Mike Capuano, a 20-year incumbent, by a 59-41 percent margin.

Pressley had three times “topped the ticket” in at large Boston city council contests, but was a newcomer to the national stage.  Now she is poised to become the first black woman to represent Massachusetts in Congress.  (Hers will be the only name on the November ballot; no Republican is running.)

Few issues separated Pressley, 44, and Capuano, 66, one of the most liberal members of the already liberal Massachusetts delegation.  But Pressley’s message of “change can’t wait” resonated with voters frustrated by the pace and direction of Washington politics.  The result was surprisingly large turnout:  in a primary held immediately after the Labor Day holiday, more than twice as many voters went to the polls as had two years earlier.

The outcome could not be explained purely in terms of identity politics.  Non-Hispanic whites are a minority in the district, but they make up 55% of registered voters.  During the campaign, Pressley tred carefully, urging voters to value her different “lived experiences” (e.g., being raised by a single mother), but not promoting her candidacy solely in terms of race or gender.

Her upset victory was not echoed in the state’s other congressional primaries, where two more “progressive” minority women tried to unseat Democratic incumbents.  Reps. Richard Neal and Stephen Lynch both dispatched their own challengers, Tahirah Amatul-Wadud and Brianna Wu, by margins of 2:1 and 3:1, respectively.

In the state’s other major congressional race, as many as 13 candidates entered the Democratic primary to succeed outgoing U.S. Rep. Nikki Tsongas in the Third District.  Ten remained by primary day, but when the smoke cleared that tally had been narrowed only to two.  For now, Lori Trahan appears to have eked out a 52-vote victory over Dan Koh, although provisional ballots and a likely recount could overturn that result.  A recount is all-but-certain, so the real winner will not be known for weeks.

In a diverse field, both Trahan and Koh are seasoned professionals.  Trahan served as chief of staff under a former congressman from that district, Marty Meehan, while Koh served as chief of staff to Boston’s current mayor, Marty Walsh.  Whoever ultimately wins will face Rick Green, a Pepperell businessman who ran unopposed for the Republican nomination — and who will enjoy a head start over whoever his opponent turns out to be.