Commentary Brian Murray knows just how fierce an opponent Arizona’s Sen. Kyrsten Sinema can be. The Republican strategist saw his candidate lose to Democrat Sinema in their 2012 race for the House. Calling the experience “unpleasant,” Murray admits his candidate, Vernon Parker, was flawed, but “flawed candidates win all of the time. Kyrsten, however, was an absolute machine.” Sinema, along with fellow centrist Democrat Sen. Joe Manchin of West Virginia, has recently become the epicenter of American politics largely because she insists on siding with the interests of her constituents instead of the demands of her party. Over the past few weeks, she has refused to support the Democrats’ Build Back Better bill—a social spending package with an initial $3.5 trillion price tag (now $2 trillion) to fight climate change, fund child care and universal pre-K, and extend the expanded child tax credit. Progressives saw her stance as a betrayal, …