(a) Primary election means an election held by a State or a political party, including a run-off election, or a nominating convention or a caucus—

(1) For the selection of delegates to a national nominating convention of a political party;

(2) For the expression of a preference for the nomination of Presidential candidates;

(3) For the purposes stated in both paragraphs (a) (1) and (2) of this section; or

(4) To nominate a Presidential candidate.

(b) If separate primary elections are held in a State by the State and a political party, the primary election for the purposes of this subchapter will be the election held by the political party.