A capital gains tax cut will generate revenue because investors will bring forward the realization of capital gains in anticipation that the capital gains tax rate will be raised when party control of government changes.
That’s essentially what happened with the Carter capital gains cut.
Also that rate cut was substantial enough to encourage people who hadn’t considered selling assets to take advantage of the opportunity.
In the Carter years the Republican party hadn’t endorsed the idea of cutting taxes to promote economic growth. That policy was up for grabs with a couple of prominent Democratic Senators endorsing the idea, along with a few GOP outliers. On the Democrat side IIRC it was Russell Long and Lloyd Bentson, on the GOP it was Jack Kemp and potential candidate Ronald Reagan.