                               In the

    United States Court of Appeals
                 For the Seventh Circuit
                     ____________________

No. 16-3457
COOK COUNTY REPUBLICAN PARTY and CHICAGO REPUBLICAN
PARTY,
                                   Plaintiffs-Appellees,
                                 v.

FRANCES SAPONE and SAMMY TENUTA,
                                             Defendants-Appellants.
                     ____________________

         Appeal from the United States District Court for the
           Northern District of Illinois, Eastern Division.
             No. 16 C 6598 — Milton I. Shadur, Judge.
                     ____________________

  ARGUED FEBRUARY 15, 2017 — DECIDED SEPTEMBER 7, 2017
                ____________________

   Before BAUER, EASTERBROOK, and HAMILTON, Circuit
Judges.
    EASTERBROOK, Circuit Judge. This suit began as Cook Coun-
ty Republican Party and Chicago Republican Party v. Cook Coun-
ty Board of Election Commissioners. The Party (as we call the
two parties collectively) contended that the Board must in-
clude on the ballot a candidate that the Party slated for the
House of Representatives in the November 2016 election. But
2                                                   No. 16-3457

the Board had never announced a plan to exclude the candi-
date. The district court entered an injunction compelling the
Board to keep this candidate on the ballot. The court did not
ask whether there ever had been a case or controversy be-
tween the Party and the Board. 198 F. Supp. 3d 886 (N.D. Ill.
2016). The Board did not appeal.
     The Party named two additional defendants: Frances
Sapone and Sammy Tenuta. In March 2016 each had been
elected a ward committeeman, a position that includes a seat
on the Party’s central committee. The Party, which refused
to seat them, sought a declaratory judgment that its action is
valid under Illinois law. Its refusal to seat them is what led it
to worry that the Board would not put its candidate on the
ballot. The Party feared that the Board would deem the se-
lection invalid because the central committee was not
properly constituted. That worry proved to be unfounded,
but the Party’s dispute with Sapone and Tenuta remains
live. It is not, however, itself a federal claim—the Party’s po-
sition against Sapone and Tenuta rests on state law, and the
litigants are not of diverse citizenship.
    The state-law dispute is a serious one. Illinois law pro-
vides for the election of ward committeemen. 10 ILCS 5/7-
8(b). Sapone and Tenuta won their elections and contend
that they are entitled to be seated unless disqualified under
10 ILCS 5/7-8(k) (felony conviction) or 5/8-5 (death, resigna-
tion, or moving outside the ward of election). Sapone and
Tenuta describe these statutory conditions as the sole
grounds of disqualification. But the Party has eligibility
rules. One of these is that a person is ineligible to be a ward
committeeman if he voted in any other party’s primary with-
in eight years of standing for election as a ward committee-
No. 16-3457                                                     3

man. (This rule was adopted less than two weeks before the
election in which Sapone and Tenuta ran, but they do not
make anything of this timing.) Sapone had voted in every
Democratic primary during the eight years preceding her
election as Republican ward committeeman, see 223 F. Supp.
3d 713, 715 (N.D. Ill. 2016), and Tenuta had voted in at least
one Democratic primary during those years. The Party con-
tends that its eligibility rules are valid under Illinois law.
The Party adds an anticipatory federal contention: if state
law does not respect the Party’s eligibility rules, then Illinois
violates the First Amendment. We call this anticipatory be-
cause (a) neither the Cook County Board of Election Com-
missioners nor any state official has suggested that the Par-
ty’s eligibility rules are invalid, and (b) it is just a potential
response to a potential contention by Sapone and Tenuta.
    The district court ruled in the Party’s favor on its conten-
tion that its rules are valid under Illinois law and added that,
if local or state officials nonetheless were to contest the Par-
ty’s rules, then the officials would be violating the Constitu-
tion. 207 F. Supp. 3d 841 (N.D. Ill. 2016). For the constitu-
tional part of its decision the district court relied on Eu v. San
Francisco County Democratic Central Committee, 489 U.S. 214
(1989), which holds that political parties have a right under
the First Amendment to choose their own leaders. The judge
did not consider the fact that public officials were not con-
testing the Party’s claims—recall that the Board did not ap-
peal and that the relief against it became incontestable when
the time for appeal expired. See 1000 Friends of Wisconsin Inc.
v. United States Department of Transportation, 860 F.3d 480 (7th
Cir. 2017). Nor did the judge consider the possibility that he
was issuing an advisory opinion.
4                                                   No. 16-3457

    That the Party’s claim against Sapone and Tenuta rests
on state law and the Party’s own rules is the initial jurisdic-
tional problem as the case reaches us. The Party’s claim
against the Board depended on federal law, which raises the
possibility that its claim against Sapone and Tenuta could
come under the supplemental jurisdiction. But 28 U.S.C.
§1367(a) limits the supplemental jurisdiction to events “so
related to claims in the action within such original jurisdic-
tion that they form part of the same case or controversy un-
der Article III of the United States Constitution.” The Party’s
claim against the Board demanded the inclusion on the bal-
lot of a candidate for the House of Representatives in No-
vember 2016. The Party’s claim against Sapone and Tenuta
denies that all elected ward committeemen must be seated
on the Party’s central committee. The validity of the Party’s
rule potentially matters to both claims, but they are not re-
motely a single case or controversy under Article III.
    This leaves the question whether the Party’s claim
against Sapone and Tenuta arises under federal law and
therefore can support original federal subject-matter juris-
diction. There is a federal issue, but it is a defense; the Party
submits that its rule is valid and that it need not seat Sapone
and Tenuta. They, too, rely on state law, not on the Constitu-
tion. The federal issue would matter in potential reply to an
argument Sapone and Tenuta might make. That’s not
enough to have a claim “arise under” federal law, which is
required for original jurisdiction under 28 U.S.C. §1331.
   Declaratory-judgment suits under 28 U.S.C. §2201 can
complicate the ascertainment of subject-matter jurisdiction
by casting a natural defendant as the plaintiff. That’s what
happened here; the Party sued Sapone and Tenuta to defend
No. 16-3457                                                   5

its decision to exclude them, rather than waiting for them to
assert a right to be seated on the central committee. The Su-
preme Court has told us that the best way to evaluate juris-
diction in a declaratory-judgment suit is to determine
whether the mirror-image suit by the other side would be
within federal jurisdiction. See Franchise Tax Board v. Con-
struction Laborers Vacation Trust, 463 U.S. 1, 19 (1983). So let
us try that exercise.
    If Sapone and Tenuta had sued the Party, demanding
membership on its central committee, their claim would
have arisen under Illinois law: they received the most votes
in elections as ward committeemen, who get seats on the
central committee. The Party would have defended by rely-
ing on its rule. Sapone and Tenuta would have rejoined that
the rule is invalid—that 10 ILCS 5/7-8(k) and 5/8-5 are the
sole exceptions to the seating of elected officials. Only then,
in the fourth round of pleadings, would the Party have con-
tended that, if Sapone and Tenuta are right about Illinois
law, the state violates the First Amendment by abridging a
political party’s right to choose its leaders. A claim does not
“arise under” a fourth-tier line of argument in a suit that is
fundamentally about state law and a private organization’s
bylaws; even a federal defense (the second tier) does not
make a claim arise under federal law. See Skelly Oil Co. v.
Phillips Petroleum Co., 339 U.S. 667, 672 (1950); Louisville &
Nashville R.R. v. Mottley, 211 U.S. 149 (1908); South Bend v.
South Bend Common Council, 865 F.3d 889 (7th Cir. 2017). That
the district court decided this dispute in the Party’s favor
under Illinois law, and did not need to mention the First
Amendment, drives the point home.
6                                                 No. 16-3457

   The district court should not have adjudicated the dis-
pute among the Party, Sapone, and Tenuta. The declaratory
judgment is vacated, and this aspect of the case is remanded
with instructions to dismiss for lack of subject-matter juris-
diction.
