Federal courts have struck down gerrymanders on racial grounds, but not on grounds that they unfairly give advantage to a political party — the more common form of gerrymandering. The case could now go directly to the Supreme Court, where its fate may rest with a single justice, Anthony M. Kennedy, who has expressed a willingness to strike down partisan gerrymanders but has yet to accept a rationale for it.

Should the court affirm the ruling, it could upend the next round of state redistricting, in 2021, for congressional and state elections nationwide, most of which is likely to be conducted by Republican-controlled legislatures that have swept into power in recent years.

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Several election-law scholars said the ruling was especially significant because it offered, for the first time, a clear mathematical formula for measuring partisanship in a district, something that had been missing in previous assaults on gerrymandering.

The 2-to-1 ruling by the United States District Court for the Western District of Wisconsin said that the Legislature’s remapping violated both the First Amendment and the Equal Protection Clause of the 14th Amendment because it aimed to deprive Democratic voters of their right to be represented. “Although Wisconsin’s natural political geography plays some role in the apportionment process,” the court wrote, “it simply does not explain adequately the sizable disparate effect” of Republican gains in the State Assembly after the boundaries were redrawn.