Dan Janison Melville. N.Y. Tuesday January 26, 2010. Daniel Janison,

Dan Janison has been a reporter at Newsday since 1997.

If you don’t think it matters which party wins the White House, keep in mind that presidents pick judges for the nation’s highest court.

Consider that the Supreme Court is preparing to rule sometime by June in a case that has a number of big public-service unions, which tilt Democratic, bracing for a huge setback.

The case, Friedrichs v. California Teachers Association, will determine whether employees can be required to pay for representation in labor organizations they choose not to join.

Precedent for “agency shops” could be reversed and unions denied their current levels of automatic income, possibly sapping their political clout.

And whether you hate that or love it is beside the point. Republican presidents picked the current court majority that’s taken up the case.

Another pending high-court case — argued in December and also awaiting a Supreme Court ruling — carries even sharper partisan implications.

At stake in that case, Evenwel v. Abbott, is the definition of “one person, one vote.”

The case comes out of Texas, backed by conservative activist Edward Blum. His group Project on Fair Representation has also sued seeking to void a race-conscious admissions program at the University of Texas and helped strike down a portion of the Voting Rights Act.

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The Evenwel case turns on whether legislative districts should be determined based on the total number of people, as is now the practice, or the total number of eligible voters.

The political wonks who deal with these matters call the distinction big.

Lawyer Jeff Wice, a Long Beach resident, is one such wonk. He advised New York’s Democratic attorney general, Eric Schneiderman, on a “friend of the court” brief submitted against such a change in the standard.

Drawing districts based on numbers of eligible voters means excluding from consideration minors who are too young to vote and non-citizens and convicted felons who are barred from voting.

Trying to calculate their exact numbers would cause “chaos,” Schneiderman and allies warned. He called the standard sought by the plaintiffs “ill-defined in theory and unworkable in practice.”

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“It’s literally a computational nightmare because most of these numbers aren’t available,” Wice stated.

But the partisan impact can be guessed.

In New York State, a change to an “eligible voter” standard could help Republicans in the state Senate, perhaps by adding two non-urban districts upstate.

Within New York City, Brooklyn’s population is estimated to be 30 percent children, versus Manhattan’s child population, which is 9 percent — a possible advantage to the latter borough. Queens and The Bronx could be losers too, analysts say.

By another estimate, New York could lose one of its Democratic-held congressional seats.

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Wice sees the Evenwel ruling as important — perhaps the biggest on the issue since the 1960s.

For now, mostly the wonks are watching. One day, voters could feel the effect, whether they recognize it or not.