India, Feb. 2 -- A political unknown, Democrat Taylor Rehmet, scored a decisive runoff victory Saturday for a Texas state Senate seat, defeating Republican Leigh Wambsganss, who was endorsed by President Donald Trump, by over 11 points. The upset in a district held by Republicans for over three decades has instantly transformed Rehmet into a national figure and ignited intense speculation about what his win truly signifies.

Rehmet, an aircraft mechanic and union leader, will only serve until early January, setting up a high-stakes November rematch for the full term. His unexpected triumph has analysts and party operatives scrambling to decipher whether it was a perfect storm of local factors or an early indicator of a shifting political ...