Pennsylvania’s new congressional map will help shape political power for a decade — and Gov. Tom Wolf is laying down some rules as it gets drawn.

Wolf plays a key role in the decennial redistricting: The Republican-controlled state legislature will draw a map, but Wolf, a Democrat, can veto that legislation. That gives Democrats some leverage in drawing the state’s 17 U.S. House districts (Pennsylvania is losing one seat). And how those boundaries are drawn can help determine who gets sent to Washington, which communities have the most influence in policymaking, and even which party controls Congress.

Before maps are proposed — likely in the next few weeks — Wolf unveiled a list of principles to evaluate them.

“The decision of whether to accept or veto the upcoming map will be one of my most important moments as governor,” Wolf said in a statement last week, “and these principles will be crucial in guiding my review.”

They were drawn up by the governor’s six-member Redistricting Advisory Council. The principles are largely standard redistricting fare, written broadly.

Here’s what you should know:

Several principles are straightforward

The principles of compactness, contiguity, and population equality are widely accepted.

Districts are legally supposed to have about the same population — “one person, one vote” — and be contiguous, meaning they can’t be split across disconnected areas. And they’re often expected to be relatively compact — a way of measuring whether districts cover one blob-like area or have long tentacles and odd shapes. Compactness has traditionally been an easily understood way to evaluate a map, though more sophisticated tools have emerged.

“Compactness is a tool against gerrymandering, but it is sometimes a crude tool. … Sometimes, naturally communities aren’t very compact,” said Michael Li, a redistricting expert at the Brennan Center for Justice at New York University.

Wolf’s council also advised him to consider whether maps meet Voting Rights Act requirements for “opportunity districts” where communities of color have the power to elect candidates of their choice. (Those districts don’t necessarily have to be “majority-minority.”)

Wolf will use computer-drawn maps to help evaluate the legislature’s proposal

Computer-drawn maps have become a powerful tool for understanding political geography. Computers can randomly draw huge numbers of maps and measure their various characteristics.

Say you draw thousands of maps dividing Pennsylvania based on neutral criteria such as compactness, then count the number of districts that would have been won by Donald Trump or Joe Biden last year. A lot of maps would have a roughly even number of Biden and Trump districts. (That follows because Biden won the state by 81,000 votes, a relatively tight margin.)

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That gives you a sense of what the natural distribution should be — helping contextualize whatever map the legislature sends.

“We suggested to the governor that using previous elections — which vary, some were red, some were blue — that he look at the proportionality of the map in that context,” said Lee Ann Banaszak, a Penn State political science professor on Wolf’s redistricting council, referring to the number of districts each party would win.

A Wolf spokesperson said “comparing maps is an important tool” that will be “one of many that the governor will use.”

School districts should be kept together, the principles say

One sign of a potential gerrymander is when districts cut across political subdivisions such as counties and towns seemingly arbitrarily — suggesting the districts may have been drawn not to fairly represent the residents of those areas but to surgically divide them for the benefit of one party.

That’s why minimizing such splits is a well-established way of keeping communities together. And Wolf’s council explicitly includes school districts, not just counties and municipalities. Banaszak said that came from public input.

“We tend to think of political jurisdictions as county lines in particular,” she said. “But what we heard in … public sessions is there are other political jurisdictions that are important.”

Keeping ‘communities of interest’ together

Keeping “communities of interest” intact is one principle that nonpartisan experts cite as key in drawing fair political maps: However a community might be defined, it should be kept together so it can be properly represented.

But it’s hard to define a community.

Consider how difficult it can be to even define where one Philadelphia neighborhood ends and another begins. And there are many ways to distinguish regions. You can use economic factors, such as which industries residents work in. Or you might use social and cultural factors, even something as seemingly superficial but powerfully regional as sports teams.

How you prioritize principles makes a big difference

Two closely related principles are more complicated than they might seem. The first says maps should produce results “proportional to statewide voter preference,” and the second says maps should have “districts where partisan swings were reflected in changes in the congressional delegation.”

Those ideas suggest a map should be responsive to the politics of the voters: In a roughly 50-50 state such as Pennsylvania, a big-wave election should result in the winning party taking more seats — not just winning the seats it already controls by larger margins.

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But those principles can get messy if they’re weighted too heavily, especially amid polarization that has produced deep-blue cities, deep-red rural areas, and increasingly divided suburbs.

“Sometimes, the only way you get competition is that you violate communities, because communities are sometimes politically homogenous,” Li said.

Philadelphia, for instance, is so heavily Democratic that drawing competitive districts there would require cracking the city into many districts that expand into sprawling exurbs. Competitiveness and proportionality often occur more naturally from focusing on other principles of fair districting, Li said, especially in swing states like Pennsylvania.

“You don’t really need to put your thumb on the scale for competition,” he said. Nor for proportionality: “Proportionality is a good goal, but you shouldn’t gerrymander to get there.”

Banaszak said the goal is not to emphasize competitiveness but to ensure that in politically divided areas, “you should leave them competitive, not try and turn them into an easy win for either party.”

Wolf will consider the transparency of the process

The council says the congressional map should be made with public input. Republican lawmakers have held hearings and are accepting maps from the public, but it’s not clear whether or how that might influence the final product.

“We got a lot of feedback on the draft principles that we had, but people continued to say they were worried about the process,” Banaszak said.

Republicans say this will be the most open and transparent redistricting in state history — which will likely be true, but only because of how low the bar is.

When the legislature does advance a map, the council said, lawmakers should explain their decisions, including why counties were split and which specific communities of interest were prioritized.

Wolf, the council said, “should disfavor any map that is made public and passed quickly with limited legislative debate or opportunity for public consideration.”