I just found a link to a paper written in 2003 by Geoffrey L. Cohen of Yale University.
Four studies demonstrated both the power of group influence in persuasion and people’s blindness to it. Even under conditions of effortful processing, attitudes toward a social policy depended almost exclusively upon the stated position of one’s political party. This effect overwhelmed the impact of both the policy’s objective content and participants’ ideological beliefs (Studies 1–3), and it was driven by a shift in the assumed factual qualities of the policy and in its perceived moral connotations (Study 4). Nevertheless, participants denied having been influenced by their political group, although they believed that other individuals, especially their ideological adversaries, would be so influenced. The underappreciated role of social identity in persuasion is discussed.
That's written in journal-ese, so I'll post a translation from the article I found that contained the link:
My favorite study (pdf) in this space was by Yale’s Geoffrey Cohen. He had a control group of liberals and conservatives look at a generous welfare reform proposal and a harsh welfare reform proposal. As expected, liberals preferred the generous plan and conservatives favored the more stringent option. Then he had another group of liberals and conservatives look at the same plans, but this time, the plans were associated with parties.
Both liberals and conservatives followed their parties, even when their parties disagreed with their preferences. So when Democrats were said to favor the stringent welfare reform, for example, liberals went right along. Three scary sentences from the piece: “When reference group information was available, participants gave no weight to objective policy content, and instead assumed the position of their group as their own. This effect was as strong among people who were knowledgeable about welfare as it was among people who were not. Finally, participants persisted in the belief that they had formed their attitude autonomously even in the two group information conditions where they had not.”
Also, the final study conducted had subjects write editorials either in support of or against a single policy proposal. The differences in how people responded in the "no group information" condition and the "my political party supports / opposes" conditions are also illuminating...