Mr. Price’s vote share could be construed as a sign that the district was safely Republican if he were considered to be a particularly weak incumbent. But he was a longtime member of Congress — like Mr. Reichert — and a powerful one: a committee chair considered impressive enough to become a cabinet member.

National political conditions are worse for the Republicans than they were in 2014 or 2016. Republicans now hold the presidency and the Congress, with Mr. Trump’s approval rating around 40 percent. On paper, these are the sort of conditions that tend to build up to so-called wave elections, like the ones that swept the Republicans out of power in 2006 and back into power in the House in 2010.

Put it all together, and it’s hard to avoid the conclusion that the race ought to be competitive. Based on factors that tend to predict which districts are likeliest to be competitive, like recent election results and the national environmental conditions, Republicans might be favored to win by only about five points in this race.

This is a very uncertain estimate in a number of respects. If the last two presidential elections are given equal weight, then the Republican might be favored to win by nine or 10 points.

But above all, the estimates are uncertain because special elections are hard to forecast. There’s a wider range of possibilities when turnout is low, when there isn’t a national race and when parties have less time to recruit candidates.

This Georgia race is emerging as a great example.

When the seat opened up, it was reasonable to assume that a special election would work to the advantage of Republicans. The Democrats didn’t have a candidate. And in recent years, Republicans have excelled in special elections because they’ve done well among the older and reliable voters who dominate low-turnout elections.

Instead, everything has been breaking toward the Democrats — and it probably wouldn’t have happened without a special election.