The phrase "formidable candidate" means someone who is very strong, impressive, and difficult to beat or compete against in a contest or election. Full definition
Former judge Mary Barzee Flores is switching races to challenge Republican Rep. Mario Diaz - Balart, ensuring that Democrats will field formidable candidates in all three Miami - Dade U.S. House seats held by Republicans during the 2018 election. (miamiherald.com)
But this November, she is trying again and facing an even more formidable candidate, Chuck Schumer. (spectrumlocalnews.com)