From Texapedia:
The most infamous element of the 1948 Texas Senate race centered on “Box 13,” a ballot box from Jim Wells County, Precinct 13. Johnson gained an additional 202 votes from this precinct in the final count, all added after the initial returns. These votes were highly uniform—reported in alphabetical order, cast at the end of the day, and almost exclusively for Johnson. This sudden bump provided the margin Johnson needed to overtake Stevenson.
While no court ever found Johnson personally responsible for fraud, his campaign’s coordination with local operatives in these counties raised persistent questions about the boundaries between machine politics and outright manipulation.
Add in Dem control of the courts and see what happens ...
IIRC, the man who orchestrated the 1948 steal for Johnson confessed to it shortly before his death and provided details on how it was done.