The Arizona 5th congressional district's consistent Republican margins of 16 to 21 points in recent presidential and House elections establish the core driver behind Mark Lamb's 89.5% implied probability. Lamb secured the GOP nomination in the July 21 primary with 58.7% against self-funded challenger Daniel Keenan, aided by Donald Trump's endorsement, despite prior allegations of misconduct that surfaced in Arizona Republic reporting. Elizabeth Lee captured the Democratic primary with 66% but faces structural headwinds in a district rated solid or safe Republican by multiple forecasters. Lamb's $1.7 million fundraising edge and the open-seat dynamic following Andy Biggs's gubernatorial bid further reinforce trader consensus on a likely GOP hold ahead of the November 3 general election.