In the last state primary of 2020, Senator Chris Coons of Delaware faces a progressive challenger.

Shortly after the 2018 midterm elections, progressive groups looking at the 2020 Senate map alighted on one state where they had a chance to contest a safely Democratic seat against a centrist incumbent: Delaware.

But a bid to oust Senator Chris Coons never became a cause célèbre on the left. Liberal groups instead focused on House races, where they won key primary victories over veteran congressmen in Chicago, St. Louis and the Bronx.

And today Mr. Coons, a 10-year incumbent, is the clear favorite in a primary against a progressive challenger, Jessica Scarane. A poll found him leading by 40 percentage points, a margin sufficient to dissuade groups from spending money to help Ms. Scarane.

Still, Mr. Coons has used an enormous fund-raising advantage to blanket Delawareans with television ads, spending nearly $800,000, compared with Ms. Scarane’s $65,000. The only third-party organization to devote significant resources to the race has been the American Chemistry Council, which aired more than $200,000 in ads backing Mr. Coons.