Jeffrey Rosen asks "Why are conservatives, not liberals, fixated on amending the Constitution?" He claims that the answer is "Populism." Maybe, but a simpler answer might be that liberals already largely have the Constitution that they want.
Rosen notes that "Conservatives didn’t always dominate the market in constitutional amendments. During the last wave of progressive constitutionalism, at the beginning of the twentieth century, Harvard Law graduates like Louis Brandeis and muckraking journalists like Ida Tarbell helped mobilize grassroots support for trust busting and anti-monopoly laws by vilifying reckless bankers and oligarchs like J.P. Morgan and J.D. Rockefeller who took risks with “other people’s money.” At the same time, progressives pursued similar anti-corporate goals by successfully championing constitutional amendments that authorized a federal income tax and the direct election of senators.
But, as Alan Brinkley has argued, liberalism began to move away from this populist spirit in the middle of the New Deal, and, in the process, largely abandoned its interest in amending the constitution."
Might I suggest the issue was not a movement away from populist spirit, but that liberals largely got the Constitution that they wanted? In the late nineteenth and early twentieth century, the Commerce Clause and economic substantive due process meant something. It put real constraints on legislatures enacting the kind of regulation that progressives wanted. In 1937, in the West Coast Hotel case, the court abandoned that. The legislature became much freer to enact economic regulation. Progressives realized that they had what they wanted - and so they abandoned the project. And, although those precedents have been chipped away at some, the liberals still largely have the Constitution that they want. Conservatives and libertarians don't, which is why we want to change it.
Rosen's comments on the ERA are largely in line with my thesis as well -- once the feminists got the decisions they wanted, throwing their energies into pushing for an amendment seemed less attractive.
*The title's tongue in cheek, of course, but the point is that movements turn to constitutional amendments when they fail to get what they want through constitutional litigation.