diff --git a/altair/vegalite/v5/api.py b/altair/vegalite/v5/api.py
index 26415c5f8..b6ce8b66f 100644
--- a/altair/vegalite/v5/api.py
+++ b/altair/vegalite/v5/api.py
@@ -513,7 +513,7 @@ def check_fields_and_encodings(parameter: Parameter, field_name: str) -> bool:
 _PredicateType: TypeAlias = Union[
     Parameter,
     core.Expr,
-    Map,
+    "_ConditionExtra",
     _TestPredicateType,
     _expr_core.OperatorMixin,
 ]
@@ -538,12 +538,6 @@ def check_fields_and_encodings(parameter: Parameter, field_name: str) -> bool:
 ```
 """
 
-_ConditionType: TypeAlias = t.Dict[str, Union[_TestPredicateType, Any]]
-"""Intermediate type representing a converted `_PredicateType`.
-
-Prior to parsing any `_StatementType`.
-"""
-
 _LiteralValue: TypeAlias = Union[str, bool, float, int]
 """Primitive python value types."""
 
@@ -560,15 +554,15 @@ def _is_test_predicate(obj: Any) -> TypeIs[_TestPredicateType]:
     return isinstance(obj, (str, _expr_core.Expression, core.PredicateComposition))
 
 
-def _get_predicate_expr(p: Parameter) -> Optional[str | SchemaBase]:
+def _get_predicate_expr(p: Parameter) -> Optional[_TestPredicateType]:
     # https://vega.github.io/vega-lite/docs/predicate.html
     return getattr(p.param, "expr", Undefined)
 
 
 def _predicate_to_condition(
     predicate: _PredicateType, *, empty: Optional[bool] = Undefined
-) -> _ConditionType:
-    condition: _ConditionType
+) -> _Condition:
+    condition: _Condition
     if isinstance(predicate, Parameter):
         predicate_expr = _get_predicate_expr(predicate)
         if predicate.param_type == "selection" or utils.is_undefined(predicate_expr):
@@ -595,12 +589,12 @@ def _predicate_to_condition(
 
 
 def _condition_to_selection(
-    condition: _ConditionType,
+    condition: _Condition,
     if_true: _StatementType,
     if_false: _StatementType,
     **kwargs: Any,
-) -> SchemaBase | dict[str, _ConditionType | Any]:
-    selection: SchemaBase | dict[str, _ConditionType | Any]
+) -> SchemaBase | _Conditional[_Condition]:
+    selection: SchemaBase | _Conditional[_Condition]
     if isinstance(if_true, SchemaBase):
         if_true = if_true.to_dict()
     elif isinstance(if_true, str):
@@ -614,17 +608,18 @@ def _condition_to_selection(
         else:
             if_true = utils.parse_shorthand(if_true)
             if_true.update(kwargs)
-    condition.update(if_true)
+    cond_mutable: Any = dict(condition)
+    cond_mutable.update(if_true)
     if isinstance(if_false, SchemaBase):
         # For the selection, the channel definitions all allow selections
         # already. So use this SchemaBase wrapper if possible.
         selection = if_false.copy()
-        selection.condition = condition
+        selection.condition = cond_mutable
     elif isinstance(if_false, (str, dict)):
         if isinstance(if_false, str):
             if_false = utils.parse_shorthand(if_false)
             if_false.update(kwargs)
-        selection = dict(condition=condition, **if_false)
+        selection = _Conditional(condition=cond_mutable, **if_false)  # type: ignore[typeddict-item]
     else:
         raise TypeError(if_false)
     return selection
@@ -785,7 +780,7 @@ def _parse_when(
     *more_predicates: _ComposablePredicateType,
     empty: Optional[bool],
     **constraints: _FieldEqualType,
-) -> _ConditionType:
+) -> _Condition:
     composed: _PredicateType
     if utils.is_undefined(predicate):
         if more_predicates or constraints:
@@ -842,7 +837,7 @@ def _parse_otherwise(
 
 class _BaseWhen(Protocol):
     # NOTE: Temporary solution to non-SchemaBase copy
-    _condition: _ConditionType
+    _condition: _Condition
 
     def _when_then(
         self, statement: _StatementType, kwds: dict[str, Any], /
@@ -866,7 +861,7 @@ class When(_BaseWhen):
     `polars.when <https://docs.pola.rs/py-polars/html/reference/expressions/api/polars.when.html>`__
     """
 
-    def __init__(self, condition: _ConditionType, /) -> None:
+    def __init__(self, condition: _Condition, /) -> None:
         self._condition = condition
 
     def __repr__(self) -> str:
@@ -1129,7 +1124,7 @@ class ChainedWhen(_BaseWhen):
 
     def __init__(
         self,
-        condition: _ConditionType,
+        condition: _Condition,
         conditions: _Conditional[_Conditions],
         /,
     ) -> None:
@@ -1710,7 +1705,7 @@ def condition(
     *,
     empty: Optional[bool] = ...,
     **kwargs: Any,
-) -> dict[str, _ConditionType | Any]: ...
+) -> _Conditional[_Condition]: ...
 @overload
 def condition(
     predicate: _PredicateType,
@@ -1719,7 +1714,7 @@ def condition(
     *,
     empty: Optional[bool] = ...,
     **kwargs: Any,
-) -> dict[str, _ConditionType | Any]: ...
+) -> _Conditional[_Condition]: ...
 @overload
 def condition(
     predicate: _PredicateType, if_true: str, if_false: str, **kwargs: Any
@@ -1732,7 +1727,7 @@ def condition(
     *,
     empty: Optional[bool] = Undefined,
     **kwargs: Any,
-) -> SchemaBase | dict[str, _ConditionType | Any]:
+) -> SchemaBase | _Conditional[_Condition]:
     """
     A conditional attribute or encoding.