Tim Carney is mostly right when he says this:
The sands shift, and there are exceptions, but I think the general rule stands: Grassroots of both parties tend to be more nationalist, while the elites of both parties are far more internationalist. In fact, I think there are few lines that intersect more perpendicularly to the LEFT/RIGHT axis and fall more cleanly along the ELITE/GRASSROOTS line than internationalism vs nationalism.
It’s true that most rank-and-file members of both parties are more skeptical of the benefits of trade agreements than party leaders, but on the whole Republican voters have no way to convey their dissatisfaction with these trade agreements. Skepticism about trade agreements receives virtually no hearing inside the GOP, and most would-be “populist” Republican politicians are no more interested in representing those views than the “establishment” or “elite” figures in their party. The same is more or less true for immigration and Democrats. There are Democratic voters unhappy with current immigration policy, but their views remain underrepresented or unrepresented at the state and national level.
On foreign policy, it is more accurate to say that Democratic candidates campaign more as internationalists than nationalists and then govern more as nationalists, because their core constituencies are relatively less nationalist than the general public. National Republican politicians try to have it both ways on foreign policy. On the one hand, they are happy to denounce the U.N. and various foreign governments, which costs them nothing and solidifies partisan support, but they are also quick to define their American nationalism in terms of hegemonism and global “leadership.” There is no question that this is something initially driven by party leaders and pundits, but it has come to define how most rank-and-file Republicans seem to view foreign policy issues. The merits and flaws of particular policies may or may not be important to these voters, but it is important that Republican candidates be seen insisting on the exercise of international “leadership.”
Of course, some of this will depend on what we mean by internationalism and nationalism. The last decade of prolonged conflicts and Bush-era incompetence has affected the parties’ alignments to some extent. Rhetorically, Romney is far and away the more nationalist of the two major party candidates, and that is also reflected to some degree in his expressions of disdain for diplomatic engagement with rival or unfriendly states, but he also feels compelled to be more aggressive in calling for U.S. “leadership” in Syria or in connection with Iran’s nuclear program. While calling for more assertive American “leadership” in response to a conflict could be perceived as a nationalist move, in the Syrian case it goes against what the public wants and it is only tenuously connected to any definable national interest. Republican rhetoric has associated national “strength” with global “leadership” for such a long time that any hint that a candidate might not pursue the latter enthusiastically enough is treated as proof that he favors national “decline.” Most of Romney’s likely voters expect him to be an internationalist, but they also expect him to be a more aggressive one.