Variable: isearch-lax-whitespace

If non-nil, a space will match a sequence of whitespace chars.
When you enter a space or spaces in ordinary incremental search, it
will match any sequence matched by the regexp defined by the variable
`search-whitespace-regexp'. If the value is nil, each space you type
matches literally, against one space. You can toggle the value of this
variable by the command `isearch-toggle-lax-whitespace'.