What has been the consistent theme of the Democratic Party since it's founding?

[sparkey]

As xkcd wonderfully demonstrates with their chart using DW-NOMINATE, the Democrats have pretty much always voted to the left on economic issues, even though what sorts of policies constitute being to the left, how ideological the parties are, and the relative importance of economic issues have fluctuated A LOT over time. So options 1 and 2 are both reasonably true, with caveats. Between the two, option 1 is probably closer to the Democrats' historical focus.

I know it's not an option, but haven't the Democrats also been the preferred party of recent immigrants throughout their history? Has there been a time when immigrants preferred Republicans?

[sparkey]

It's a great chart, but DW-NOMINATE just measures how much distance there was between the positions (voting records) of the different parties. It doesn't measure what those positions meant in ideological terms.

Well DW-NOMINATE measures how much members of Congress vote together, but the results can be broken down into 2 dimensions, the weightier one being the first dimension, which they say "can be interpreted in most periods as government intervention in the economy or liberal-conservative in the modern era." And it turns out that the Democrats have pretty much always been on the left, hence why the xkcd graphic groups them consistently under "left-leaning parties."

Maybe this is a clearer graphic to demonstrate that, although it only goes back to 1879: http://voteview.com/images/House_and_Senate_Means_1879-2012_1.jpg