These fundamentals provide only a rough sense of what might happen in an election year. But in this case, they seem to line up with everything else.

Special Elections: Great for Democrats

In the special election in Arizona on Tuesday, the Republican, Debbie Lesko, won by just five points in a district that Donald J. Trump won by 21 points . There were no excuses for Republicans: The candidates were typical, the turnout was typical, and the G.O.P. fought hard for the district.

On average, Democrats have run 14 points ahead of a district’s partisanship (as measured by the last two presidential elections, compared with the national popular vote) in special elections for Republican-held districts so far this cycle. They’ve run more than 20 points ahead on three occasions — Kansas’ Fourth, Pennsylvania’s 18th and Arizona’s Eighth — and that doesn’t include Doug Jones’s victory in the Alabama Senate race.

These Democratic over-performances are a startling departure from the Obama years, when congressional election results polarized along national political lines.

Over the more than 1,000 special and general House elections in Democratic-held districts in the Obama era, there were only four elections when the Republicans ran 20 points ahead of the district’s lean in presidential elections. This cycle’s Democrats have pulled it off three times out of seven.

In a broader historical context, though, the Democratic over-performance is not quite as startling. It is still impressive, but the Democrats ran 20 points ahead of a Republican-held district’s presidential partisanship in 31 races combined in 2006 and 2008 .

Over all, the Democrats’ performance in 2018 special congressional elections looks a lot like their showing in open districts in 2006, and well above the average from wave elections in 1994, 2006, 2008 and 2010. On average, Democrats ran 14 points ahead of a district’s partisanship in open races in 2006 — exactly the same as the Democratic over-performance so far this cycle. The Democrats had a similar 10-point over-performance in 2008.