Incumbent Republican Brian Mast holds a strong structural edge in Florida’s 21st District through incumbency, superior fundraising exceeding $5 million, and the area’s R+16 partisan lean, yet traders see the margin clustered around 10-15 points with Democrat James Martin at roughly 12% implied win probability. A late-September poll initially showed Mast ahead 49-38 but tightened to a statistical tie after candidate information was provided, highlighting vulnerability to turnout and messaging on issues such as insurance costs and Everglades restoration. With the November 3 election days away, separation could emerge from final get-out-the-vote efforts, independent spending, or any last-minute national political developments that alter voter enthusiasm in this Treasure Coast district.