Between July 21 and July 29, 2024, the Polymarket prediction market saw a notable decline in the probability of the Republican Party winning the Presidency and both houses of Congress, dropping from $0.41 to $0.33. A key event driving this shift was President Joe Biden's announcement on July 21 that he would not seek re-election, citing age-related concerns after a poor debate performance. This decision boosted Vice President Kamala Harris as the Democratic front-runner, potentially unifying Democratic support and affecting the Republican prospects .
Additionally, during this period, polling data revealed that Harris was performing competitively against former President Donald Trump, who is the presumptive Republican nominee. For instance, a Forbes/HarrisX poll from late July showed Trump leading Harris by a narrow margin of 51% to 49%, indicating a tight race that could influence market perceptions of the overall Republican chances. These developments likely contributed to the decreased confidence in a Republican sweep in the 2024 elections.