The history statistic that could give hope to Senate Democrats

The other side: As Goedert writes, "The allocation of two Senate seats regardless of state population, giving disproportionate power to small states, seems naturally stacked against the Democratic Party."

Between the lines: Open seats are traditionally easier to flip than those with an incumbent. Republicans have won 60% of open Senate seats in that same time — even when Democrats held it previously.

By the numbers: Of the 238 seats with a Democratic incumbent, a Democrat only lost 19 of them (8% turnover rate). For the 249 held by Republicans, the GOP lost 39 seats (a 16% turnover).

• Goedert also found a 3-point difference in vote share between Democratic and Republican incumbents over the last three decades.

• The 611 races analyzed includes six elections for each Senate seat, as well as a handful of special elections.
• Republicans won 30 of the 67 open seats previously held by Democrats; Democrats flipped just 14 of the 57 previously held by the GOP.

What to watch: The 2018 election and whether those 10 red-state Democrats can follow this trend.