Can I reuse the design of an earlier email campaign?

Yes, save it as a template.

1. Open the campaign under Marketing → Email Campaigns and go to the design step.
2. Click Save as template.

Next time:

1. On Marketing → Email Campaigns, click Templates.
2. Pick the saved template. A new draft is created from it.
3. Change the name, subject and content, then send or schedule it.

You can delete templates you no longer need from the same Templates window.