I'm looking for a data on:
- Nationwide data from 1946 on Democrat and Republican share of the national vote
- Share of the seats in Congress in the same period.
Would it be possible to collect the same data at state level (such as Texas)?
Assuming you are looking for data showing the vote share of each party for the 1946 House and Senate midterms, on state and national level, here is what I found.
Simple answer: The House of Representatives Office of the Historian has this data for every national election from 1920-2016 in original PDF form (from the Government Printing Office), including for the 1946 elections. The problem is, the PDF for 1946 is a scan of print documents in raster form (i.e., it isn't encoded in text form), so it's not optimal if you're putting it in a spreadsheet or doing other data analysis. But for quick reference, here is what I found:
That data appears to be exactly what you described as long as I interpreted it correctly. But for actual numerical data in digital form, I'd suggest looking through this list of election data sources from University of Michigan, or this list from the Library of Congress. That is probably a good starting place, depending on how specific and in what form you need the data.
As for party control in Congress through the years, see these links:
I'm sure more data available through proprietary services or in university databases, but as for what I could find on the public internet, I hope this helps.