In a representative democracy, the idea is that the representatives should support the preferences of their constituencies. This is what is happening with the Trans Pacific Partnership (TPP).
Hillary Clinton's vice presidential pick, Tim Kaine has reversed his position from supporting TPP to opposing it. He had praised TPP as recently as Thursday. He has changed his position on TPP, and good for him for doing so.
Kaine had to change his position immediately after being chosen as Clinton's VP pick. This is what happens when people organize and make their voices heard. This is the power of the progressive movement.
This is the new Democratic party — recognizing reality, and starting to listen to the voices of working people again. (Starting to, but certainly not all the way there yet.) Clinton and Kaine recognize the political necessity of opposing the agreement, but have not yet acknowledged how harmful "free trade" agreements generally are to democracy. The core idea of these agreements is that countries surrender their sovereign power to impose tariffs and other trade limitations, when determined to be in their own national interest. In other words, they agree to place the interests of corporations above the wider interests of the nation-at-large and their people.
Credibility Against Trump
Kaine's shift from supporting TPP helps Clinton's credibility on the issue. Donald Trump is running as a self-described "free-trader" who happens to oppose TPP. He says Clinton is only "pretending" to oppose TPP. It is important for Clinton that Kaine also voices opposition to TPP. Unfortunately she continues to defer to President Obama's continuing support for TPP, not yet calling on Obama to withdraw the agreement from consideration by Congress.
Roger Hickey, writing in Two Democratic Parties in Philadelphia: They Must Unite And Beat Trump. says, "Now Clinton and Kaine need to take their opposition to TPP into the campaign, showing Sanders voters – and voters in industrial states tempted to go for Trump – that they really mean it when they say they will stop the TPP."
Meanwhile, Trump's VP nominee Mike Pence still supports TPP.