Step 1In a large kettle or soup pot, cook bacon over medium heat until crispy, about 10 minutes. Remove with a slotted spoon to drain on paper towels and reserve, leaving drippings in pot.
Step 2Add onions to drippings and cook until softened but not brown, about 6 minutes. Sprinkle flour over onions and cook, stirring, 2 minutes. Whisk in clam broth. Add potatoes, oregano and bay leaf and simmer uncovered until potatoes are almost tender, 10 minutes. Add milk, tomatoes and clams and simmer uncovered over medium heat until potatoes are very tender, 5 to 10 minutes.
Step 3In a small dish, combine ketchup and baking soda and whisk mixture into chowder. Season with salt and pepper to taste. Set aside for an hour or so or refrigerate for up to 2 days.
Step 4When ready to serve, reheat chowder gently over medium-low heat. Remove bay leaf, add butter and swirl until melted. Adjust seasonings if necessary. Place crumbled crackers in bottoms of soup bowls and ladle chowder over them. Pass bacon or salt pork bits for sprinkling over top if desired.