Portugal opposed and declined Columbus's plan because the westward route to Asia was thought to be longer than the tip of Africa.
Portugal used the route at the tip of Africa to travel to Asia.
However, Christopher Columbus offered a westward route to Asia. As his plan was rejected, he went to Spain to win the Spanish Crown's approval.
After several failed attempts on the Spanish Crown, Columbus received the approval and the capital that enabled him to discover America by landing in the Bahamas, still thinking that he had reached East Asia.
Thus, one reason that Portugal opposed and declined Columbus's plan was that the westward route to Asia was thought to be longer than the tip of Africa.
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