(CNN) Two of Chief Justice John Roberts' top priorities are on a collision course as the Supreme Court nears a pivotal decision this week on the Trump administration's design for the 2020 census .

Roberts has repeatedly declared that he wants the public to see the court as a nonpartisan institution, even though polarizing cases have often divided it between the five justices appointed by Republican presidents and the four appointed by Democrats.

Roberts over the years has shown he's uneasy with decisions on big cases that routinely align the court, in effect, along those party lines -- an instinct that was most notably evident when he joined with the court's four Democratic-appointed justices to mostly preserve the Affordable Care Act in 2012. "We do not sit on opposite sides of an aisle, we do not caucus in separate rooms, we do not serve one party or one interest, we serve one nation," Roberts insisted in a speech last October

But on cases affecting the core electoral interests of the two parties -- like the decision impending this week on whether the Trump administration can add a citizenship question to the 2020 census -- Roberts has conspicuously deviated from that pattern. He has repeatedly joined in 5-4 Supreme Court decisions that align all the Republican-appointed justices against all the Democratic-appointed ones on cases that set the underlying rules of political competition, from campaign finance to voting rights. And on those cases, critics say, he has consistently voted with the other GOP-appointed justices to produce rulings that benefit the Republican Party's electoral interests.

"One thing Roberts has been consistent about is his willingness to take extreme stances to undermine long-standing rules of democracy," says Michael Waldman, president of the Brennan Center for Justice at New York University, a group that advocates for expanded voting rights. "We all are impressed by his institutionalism and his deftness at steering the Supreme Court away from making it such a central partisan topic, such as in the ACA case. The democracy issues are where the court has been most aggressive, and he has not been a temporizing swing vote but an ardent activist."

The census case , Department of Commerce v. New York, crystallizes these issues even more than the earlier electoral decisions in Roberts' tenure because his role on the court has changed. With the retirement of Anthony Kennedy, a President Ronald Reagan appointee who often functioned as the court's swing vote, and his contentious replacement last year by the more reliably conservative Justice Brett Kavanaugh , Roberts has become the closest thing to a swing vote on the court, legal analysts agree. That means a party-line decision supporting the Trump administration on the census case would more clearly bear his stamp -- and thus more directly undercut his attempts to portray the court as nonpartisan.

Richard Hasen, a law professor at the University of California at Irvine who specializes in election law, says that "historically ... it has been mostly true" that Roberts has consistently voted to uphold the GOP's core interests in cases that affect the rules of politics. But, Hasen says, "He's in a different position now. He's been the chief justice for a decade but only now is he a swing justice and only now is he on a court when all the conservatives are Republicans and all the liberals are Democrats."

Citing the formulation in the recent biography of Roberts by CNN legal analyst Joan Biskupic , Hasen says the census case "will be a great test ... of which Roberts is going to show up: the movement conservative Roberts or the pragmatic instituitionalist Roberts."

Roberts has spoken very little in public about specific court decisions, but in his appearance at the University of Minnesota last October, he suggested that the focus on 5-4 decisions that split the court along partisan lines was misplaced. The press, he said, portrays such decisions "more in a political way somebody must think is more readily understandable. From the court perspective, I would like to see the sentence that causes people to split this way (so) a reasonable person could see why there's two sides to that story."

Roberts also said he tries to ignore criticism of his decisions ("the good thing about life tenure is that it doesn't really bother you much") and doesn't look back wondering whether he decided cases incorrectly. "I will give the same answer because it's the true one, and I always worry it will sound harsh or something, but the answer is no," he said.

A ruling of huge consequence

The census decision due this week has potentially huge implications for the partisan -- and racial -- balance of political power over the next decade. The Trump administration is seeking to add a question that would, for the first time, ask all Americans whether they are American citizens. A wide variety of experts, including within the Census Bureau itself, have warned that the question could discourage minority groups, particularly Hispanics, from responding to the census.

The nonpartisan Urban Institute recently noted that changes in how the census is administered -- particularly an increased reliance on internet responses and reduced emphasis on home visits -- could swell the undercount of minorities. Adding a citizenship question to those changes, especially in an atmosphere of intense political tension over immigration, could produce "an undercount of a magnitude we haven't seen in a decade or two or more," says Diana Elliott, a senior research associate at the institute.

Specifically, the Urban Institute projected that a census including the citizenship question could undercount Hispanics nationwide by 3.5%, while slightly overcounting whites and also significantly undercounting African Americans and more modestly missing Asian Americans.

That could undercount the population in heavily diverse states including California, Texas, New Mexico, Nevada, New York, Florida and Georgia while leading to overcounts in mostly white states including Vermont, West Virginia, Maine, New Hampshire and Montana, the group calculated.

In the 2020 reapportionment that will be based on the census results, such an undercount could shift congressional seats -- and Electoral College votes -- from diverse states mostly within the Sun Belt (particularly California, Texas, Arizona and Florida) toward more predominantly white states mostly across the Rust Belt (Ohio, Minnesota and Montana are among the states that have been projected as possible winners ).

Just as important, an undercount of minorities could shift the balance of political power within states. With less recorded population, heavily minority areas would lose representation when states draw new districts after 2020 for state legislative and congressional seats.

In several states, Republicans are discussing proposals to draw legislative and congressional districts based not on total population but the number of citizens in each area. That approach -- if ultimately passed in states and upheld by the Supreme Court -- would tilt power even more dramatically away from areas with large Hispanic populations toward mostly white communities. Recently discovered hard drives from a deceased GOP redistricting specialist, Thomas Hofeller , showed that obtaining the data required to implement such a citizen-based redistricting system was one reason he urged the Trump administration to include a citizenship question in the census.

For all these reasons, experts such as Waldman believe the census case could rival the long-term impact of the other two landmark Roberts-era cases affecting the ground rules of politics: the 2010 Citizens United v. Federal Election Commission decision eviscerating campaign finance restrictions, and the Shelby County v. Holder decision, decided six years ago Tuesday, which severely retrenched the Voting Rights Act.

"The census case ranks up there with Citizens United and Shelby County in tilting the rules of American politics," he says.

Both of those earlier landmark cases were decided on 5-4 votes, with Roberts voting with an all-Republican majority each time. At the time Citizens United was decided, the court's partisan and ideological balance was less regimented than today. That decision featured Roberts and four other Republican-appointed justices outvoting three justices appointed by Democrats and John Paul Stevens, a Gerald Ford appointee, who often voted with the court's liberal bloc.

By the time of the Shelby County decision in 2013, Elena Kagan, appointed by President Barack Obama, had replaced Stevens and the court had settled into its current ideological and partisan alignment, though with different members. On that decision, Roberts joined four other Republican appointees to outvote the four Democratic appointees.

Each of these rulings brought huge changes to American politics.

Citizens United, which defined political spending as speech, opened the door to a rapid expansion in "dark money" super PAC spending.

Shelby County had an even more seismic effect. That decision, written by Roberts, effectively invalidated the provisions of the 1965 Voting Rights Act that required states with histories of discrimination to receive federal approval ("preclearance") for changes in their election laws. With that provision stripped away, Republican-controlled states across the country have stampeded to impose new restrictions on voting, such as limiting early and absentee voting and requiring more stringent proof of identity to cast a ballot.

Other 5-4 election decisions

Roberts has joined 5-4 court decisions that have produced outcomes mostly favored by Republicans on an array of other election-related topics. "If he's got fears of seeming partisan, he's managed to compartmentalize," says Waldman. "He seems to always have a benefit for one party over another."

. Last June, the court upheld an Ohio law that aggressively purged voters from the rolls if they had not participated in recent elections, again on a 5-4 vote that pit all of the GOP-appointed justices against all of the Democratic-appointed ones

That same month the same alignment rejected a challenge of Republican-drawn Texas redistricting maps for the state Legislature and Congress made on the grounds that they discriminated against Hispanics.

In yet another 5-4 split, the court's four Democratic-appointed justices last June signaled openness to establishing rules to limit partisan gerrymandering but were unable to attract a fifth vote from any of the court's Republican-appointed justices. The court is due to decide a new round of gerrymandering cases this week.

Back in 2011, Roberts likewise wrote an opinion in which the five GOP-appointed justices outvoted the four Democratic appointees to invalidate an Arizona election law that provided extra funds to candidates who accepted public financing and faced big-spending opponents.

In 2014, Roberts wrote an opinion for the other four GOP-appointed justices invalidating federal limits on total contributions from individuals in an election cycle.

In 2015, Roberts and three other GOP-appointed justices voted to strike down an Arizona law establishing an independent commission to draw congressional district lines, but the court's liberals prevailed when Kennedy joined them to uphold the law. With Kennedy now replaced by Kavanaugh, legal experts say it's less certain that this court will continue to allow independent redistricting commissions, which nearly 20 states have now adopted

So far, three federal district courts have decided against the Trump administration's decision to add the citizenship question, ruling that the Commerce Department choice to include it violated federal procedures for reaching such a decision. Commerce Secretary Wilbur Ross initially said he sought to add the citizenship question at the request of the Justice Department -- ostensibly to improve enforcement of the Voting Rights Act. But court testimony has shown that he asked the department to make that request, and consulted with anti-immigration hard-liners in the administration in his decision.

"In the census case it's really hard to come up with a straight-faced argument in the face of evidence that while the Trump administration said they were doing this to protect Hispanic voters they were actually doing it to hurt Hispanic voters," says Hasen. "If they are going to turn a blind eye to that ... that's a sign that this is a very partisan court. This should be a 9-0 decision (against the question)."

With the disclosure of the Hofeller memos, the court could punt on the decision and send it back to lower courts to consider the new evidence -- though most observers consider that unlikely since the Trump administration has insisted it must prepare the census forms by this summer.

the parts of America least touched by the demographic changes that are hurtling the US toward a future in which minorities will constitute most of the under-18 population as soon as 2020 and a majority of the overall population around 2043, according to census projections. Beyond its immediate partisan implications, the census case stands at the forward edge of a racial dynamic that may increasingly embroil the court over the coming decade. As I've written, the five-member Republican-appointed majority was confirmed by GOP senators -- who representthe parts of America least touched by the demographic changes that are hurtling the US toward a future in which minorities will constitute most of the under-18 population as soon as 2020 and a majority of the overall population around 2043, according to census projections.

If the five GOP-appointed justices authorize a census question that dilutes the political clout of minorities, it would be only one of many decisions that could pit them against the interests of America's growingly diverse population. Those same justices have displayed much skepticism about other programs, such as affirmative action in education or constraints on law enforcement's use of race in decision-making, that most minority leaders consider essential to their communities.

Waldman says the census case illustrates how the five Republican-appointed justices could set rules for politics in the years ahead -- on issues from redistricting to voting rights -- that constrain the influence of the growing minority population, which mostly supports Democrats, and thus help Republicans retain power.

"What it means is the Supreme Court will have an even bigger thumb on the scale for the rules of politics, and it becomes a perpetual motion machine ... changing the rules of politics to help elect more Republican officials, who appoint more Republican judges," Waldman says.

Roberts has left no doubt that one of his greatest fears is that the court he leads will be perceived as nothing more than exactly that kind of cog in the Republican political machine. The explosive decision due this week will test whether that fear is enough to dissuade him from adding Department of Commerce v. New York to Citizens United and Shelby County in his legacy of landmark 5-4 party-line court rulings that have rewritten the rules of competition between Republicans and Democrats.