First, one needs to learn what history says it means to be an American. Next, one needs to decide what they believe it means to be an American. How consistent are your beliefs with history?
Next, one needs to determine what each political party claims it stands for and challenge the assumptions for how those claims are consistent with being an American and for progress.
Of course, the questions of whether progress is needed, desired, and how it looks needs to be decided, ultimately by each individual.