In Germany, there are several surveys on voter's preference ("If there were an election next sunday, which party would you vote for"), the so-called "Sonntagsfrage".
Are historical values available? I found this graphical data, but I am looking for numbers, ideally under an open license.
Just found the Overview of the Allensbach Institute, which has data at least for the current legislative period. The data can be easily scraped, see for example my R script.
This helps a bit, but if you have another answer, I will accept and upvote yours.
Wahlrecht.de provides a good list of results of the major survays. Together with the wayback machine it should give you all the available survey data.
