The disparity in ad spending reflects the political dilemma facing so many Democrats. Loath to make impeachment appear anything other than a constitutional principle, Democrats are hesitant to use aggressive persuasion tactics to make their case for supporting impeachment. They are instead revisiting popular themes that succeeded in the midterms.

Aside from Mr. Steyer, the deepest pocketed Democrats right now — presidential candidates — have barely run any advertisements around impeachment. The Biden campaign announced a new ad on Tuesday to run ahead of impeachment proceedings, but makes no mention of impeachment.

The bulk of Republican ads avoid 2020 entirely. They have been aimed more at pressuring the members themselves to vote against impeachment, and not at furthering an anti-impeachment narrative in key swing states. Still, in the past few days virtually every Democrat who was targeted has come out for impeachment.

For most House Democrats, not even a year removed from expensive midterm campaigns, dipping into their cash reserves this early is a risky move. Running in 2020, during a presidential election, is likely to drive up advertising costs. So they are left without a robust defense against a well-funded coalition of Republican super PACs and the Trump campaign.

“For Republicans, you want to get on offense against Democrats, you want to press their issues and define them early,” said Matt Gorman, a Republican strategist and former communications director at the National Republican Congressional Committee. He noted that for House races, the ability to attack early can be key in a presidential year. “The airwaves get cluttered, put your message in now.”

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National polling on impeachment has remained largely unchanged in recent weeks, reflecting the deep polarization in the national political arena. Only two Democrats have publicly announced their opposition to impeachment so far (and one, Representative Jeff Van Drew of New Jersey, is planning on switching parties after doing so). A third Democrat came out for impeachment on one of the articles but not the other.