California's 12th Senate District was born in a bipartisan political deal after the 2000 census.

The Legislature tailored the five-county district -- centered in the upper San Joaquin Valley but stretching west into the Salinas area -- for a moderate Democrat, Assemblyman Dennis Cardoza. Cardoza instead opted to run for Congress.

Democratic leaders quickly shifted to a former assemblyman with a dairy farm background, Rusty Areias, to claim a seat, but a Republican farmer, Jeff Denham, captured it instead by the narrowest of margins. He went on to serve two terms before segueing into Congress.

Despite the district's lopsided Democratic voter registration advantage, the party was frustrated again in 2010 when Republican engineer Anthony Cannella defeated a Democratic legislator, Anna Caballero.

When an independent redistricting commission did its work after the 2010 census, it kept the 12th district largely intact, expanding it a bit into Fresno County, even though other districts were undergoing big boundary changes.

Why the status quo? Any election changes affecting four California counties were subject to U.S. Justice Department oversight under the federal Voting Rights Act, and two of them, Merced and Monterey, were part of the 12th.

Therefore, as the commission later stated, "Merced and Monterey counties were combined to meet the requirements of Section 5 of the Voting Rights Act."

Last week, Section 5 was set aside by the U.S. Supreme Court, but the 12th District remains as designed in 2011. It's also one of a handful of Senate districts considered to be battlegrounds next year as Democrats seek to retain their 29-seat "supermajority" and Republicans hope to regain clout.

The identity of Cannella's Democratic challenger has yet to emerge, but it could be one of the two Democratic assemblymen who each represent roughly half of the 12th District, Adam Gray or Luis Alejo.

Cannella, meanwhile, has become something of a GOP maverick -- co-authoring Senate President Pro Tem Darrell Steinberg's union-backed bill that would punish charter cities that refuse to impose "prevailing wage" rates on construction projects, and doing his own deal on overhauling enterprise zones that most other GOP senators opposed.

Cannella says his positions on both issues reflect his beliefs, rather than a calculated effort to position himself for re-election.

"I've always been pretty moderate on labor issues," he said. "I run a union shop, and I know what it means."

He also said it was apparent that enterprise zones were doomed, and in personal meetings with Gov. Jerry Brown last week he got concessions in the successor program that benefit his high-poverty district.

Whatever his motives, those moves will enhance his prospects of winning a second Senate term next year.