The RealClearPolitics polling average has Roy Blunt (Vice Chair of the Senate Republican Conference) up by 3.4 points. That's within the margin of error of many polls with a sample size of 600 or fewer.
Blunt's challenger, Jason Kander, released a well-publicized ad of himself assembling an AR-15 blindfolded while outlining his stance on gun control.
During Hillary Clinton's widest leads thus far in post-convention presidential polls, there was talk of Missouri becoming a swing state. Although Missouri will likely stay red in the presidential race, the down-ballot shock from a Clinton victory could knock Blunt out of his seat.
Florida's 26th District
Joe Garcia won a closely contested Democratic primary (fewer than 3 percentage points) against Annette Taddeo. Garcia represented the 26th District in the 113th Congress, but was unseated by Republican Carlos Curbelo in 2014. Curbelo has refused to support Donald Trump, even comparing him to Hugo Chavez.
Notably, Florida's 26th District was one of eight congressional districts redrawn in 2015 by order of the Florida Supreme Court. The suit, brought up by a coalition led by the League of Women Voters, claimed that Florida's district map was the product of unconstitutional political gerrymandering. Florida voters added an amendment to the Florida Constitution that requires congressional districts to be drawn in a manner establishing "fairness" and to use "city, county, and geographical boundaries."
Curbelo has far deeper pockets than Garcia. As House races go, though, it's close.