double down

Short definition from Wiktionary

Verb

1. 1
   To double one's wager, particularly, the name of a specific doubling bet allowed in blackjack.
2. 2
   To significantly increase a risk, investment, or other commitment; to respond to a challenge (e.g. to an opinion) by reinforcing or extending one's position rather than moderating it.
3. 3
   Alternative form of double down.