Sentence:  Finally, just as passage of redistricting had been made a litmus test for Republicans, defeating its passage now became a litmus test for Democrats. The Democrats <WSD>drew a line in the sand</WSD>. 
Meaning of the word marked with <WSD> tags:  To indicate the threshold or level above which something will become unacceptable or will provoke a response; to create a boundary and imply or declare that its crossing will provoke a (negative) response.