Both liberals and populists support the rights of the common man, but populists also argue for the wisdom of the common man. As such, populism can feel more democratic than liberalism and can have appeal to progressives.
Yet, the record of populism is one of failure. Most of the goals of populists were right and proper things to desire, but they weren't achieved by populists. Because of this, I argue that progressives who wish to achieve change should embrace liberalism and not populism.
Most of the policies desired by populists were enacted by FDR, who was a liberal, not a populist such as Huey Long. Unlike populists, liberals are social libertarians. Democratic populists like George Wallace found the increasingly liberal-dominated post-FDR Democratic party more and more hostile to their world view and many of them eventually became Republicans.
Like populists, liberals side with labor over management. FDR made that clear by putting the power of the Federal government clearly on the side of labor. Self-identified liberals like Paul Krugman today call for strengthening the power of labor by legislation on the behalf of unions. Kos’s slogan "Democrats are the party of people who work for a living" expresses this core allegiance.
Liberals are also believers in good government; they believe that well-designed government policy can make life better for most Americans. Liberals value policy based on ideas ground in an accurate assessment of how the world works (why liberals refer to themselves as the "Reality-based community"). In contrast, populists believe in simple solutions for complex problems. From Free Silver to Fair Tax, this penchant for simple solutions to complex problems makes populists easy prey for propaganda from the Right or Left.