With 29 days to go before the midterms, the picture is contested in ways that were not obvious a year ago: some Republican districts are moving, and several Democratic ones are not.

The Senate is the likelier chamber to flip, though a handful of states will decide it. A small number of candidates in unexpected places — including a South Carolina race most Democrats treat as a long shot — have turned races that were written off.

The House is the closer contest. Democrats are competitive across many more districts, but their ceiling has repeatedly proved lower than their floor, and incumbents in unwinnable places remain in the count.

The decisive variable may be one neither party controls. Turnout among low-propensity voters in a few battlegrounds will determine whether these late moves matter or merely make the map look closer than it is.