Today the only difference between endorse and indorse is that endorse is now the preferred spelling. Formerly indorse meant ‘to sign one’s name to something’.
For example, you can indorse a check.
To endorse is to give one’s support or approval to something or somebody.
- I cannot endorse a candidate with a record like this. (= I cannot support a candidate with a record like this.)
- We had to abandon the plan because our manager refused to endorse it.
Today you can use endorse for all meanings.