- The rain in Spain stays mainly in the plain.
- The rein in Spain stays mainly in the plain.
- The reign in Spain stays mainly in the plain.
Only sentence one is correct in normal usage. Sentence two refers to a leather strap used to guide a horse and sentence three refers to a kingdom in Spain. As long as Trump is in office, rein and reign will be used to refer to him and his administration. Those of us who write about Trump should be careful to use the correct word. A spellchecker will not catch an error.
This sentence uses all three words correctly. On a rainy day, Democrats tried to rein in the excesses of the Trump reign of terror.