The U.S. Supreme Court said on Thursday that President Donald Trump's administration did not give an adequate explanation for its plan to add a citizenship question to the 2020 census, and in a separate ruling rejected efforts to rein in the contentious practice of manipulating electoral district in two highly anticipated decisions to end the current session.

The political consequences arising from the rulings in legal challenges to the census question and partisan gerrymandering could be significant, leaving lasting effects on elections for seats in the U.S. House of Representatives and state legislatures at a time of political polarization in America.

The mixed ruling on the census does not definitively decide whether the citizenship question could be added at some point.

Trump said he would see if the 2020 census could be delayed.

"I have asked the lawyers if they can delay the Census, no matter how long, until the United States Supreme Court is given additional information from which it can make a final and decisive decision on this very critical matter," Trump tweeted.

The Census Bureau has argued that the 2020 forms need to be printed as soon as possible.

A group of states including New York and immigrant rights organizations sued to prevent the citizenship question from being included in the decennial population count. Opponents have said the question would instill fear in immigrant households that the information would be shared with law enforcement, deterring them from taking part.

The census, required by the U.S. Constitution, is used to allot seats in the House of Representatives and distribute some $800 billion US in federal funds. The intent of the citizenship question, opponents said, is to manufacture a deliberate undercount of areas with high immigrant and Latino populations, costing Democratic-leaning regions seats in the House, benefiting Republicans and non-Hispanic whites.

Commerce Secretary Wilbur Ross testifies on Capitol Hill in Washington in 2018. Ross has been among the drivers of the effort to add a citizenship question to the census. (Manuel Balce Ceneta/Associated Press)

"At this point, the government does not have authority to put this question on the census form," Chief Justice Roberts wrote in the majority 5-4 opinion.

Critics of the attempt by the administration to add the question seized on internal communications made by Wilbur Ross, the commerce secretary.

Manhattan-based U.S. District Judge Jesse Furman had earlier ruled that evidence showed Ross concealed his true motives for adding the question and that he and his aides had convinced the Justice Department to request a citizenship question.

Roberts in his opinion concurred, saying Ross "acted arbitrarily and violated certain provisions of the Census Act."

Democratic Senator Charles Schumer of New York praised Thursday's census ruling:

When even this conservative Supreme Court determines that the Trump administration’s argument is odious and dishonest, you know the administration’s motivation behind adding the citizenship question to the census in the first place was an abhorrent one. —@SenSchumer

In April's argument in the case, the court's conservative justices had appeared to be inclined to rule in Trump's favour, but evidence that surfaced in May bolstered the claim that the citizenship question was aimed at furthering Republican Party political goals.

Documents created by Republican strategist Thomas Hofeller, who died last year, showed that he was instrumental behind the scenes in instigating the addition of the citizenship question. He was an expert in drawing electoral district boundaries that maximize Republican chances of winning congressional elections.

Hofeller concluded in a 2015 study that asking census respondents whether they are American citizens "would clearly be a disadvantage to the Democrats" and "advantageous to Republicans and Non-Hispanic Whites" in redrawing electoral districts based on census data.

Gerrymandering decision 'tragically wrong': Kagan

The U.S. Supreme Court on Thursday rejected efforts to rein in the contentious practice of manipulating electoral district boundaries to entrench one party in power by turning away challenges to political maps in Maryland and North Carolina.

The justices, in a 5-4 decision with the court's conservative in the majority and liberals in dissent, sided with Democratic legislators in Maryland and Republican lawmakers in North Carolina who drew electoral district boundaries, saying in a decision with nationwide implications that judges do not have the ability to curb the practice known as partisan gerrymandering.

The ruling delivered a setback to election reformers who had hoped the court would intervene over a growing trend in which parties that control state legislatures use the electoral district line-drawing process to cement their grip on power and dilute the voting power of people who support the rival party.

U.S. Supreme Court Associate Justice Elena Kagan said in her dissent the top court had abdicated its responsibilities in the gerrymandering cases. (Jim Young/Reuters)

Roberts in his conclusion, stated: "The fact that such gerrymandering is 'incompatible with democratic principles' does not mean that the solution lies with the federal judiciary."

In a dissent, Justice Elena Kagan argued the ruling was "tragically wrong."

"Of all times to abandon the court's duty to declare the law, this was not the one," said Kagan.

The practices "imperil our system of government" and jeopardize free and fair elections, she continued.

The high court previously had struggled to resolve the legality of partisan gerrymandering, a longstanding practice in which boundaries of legislative districts are reworked with the aim of tightening one party's grip on power. The justices in June 2018 failed to issue definitive rulings on partisan gerrymandering in two cases — this same one from Maryland and another involving a Republican-drawn electoral map in Wisconsin.

The boundaries of legislative districts across the country are redrawn to reflect population changes contained in the census conducted by the federal government every decade, a head count mandated by the U.S. Constitution.

This redistricting in most states is carried out by the party in power, though some states in the interest of fairness assign the task to independent commissions. Gerrymandering typically involves politicians drawing legislative districts to pack voters who tend to favour a particular party into a small number of districts to diminish their statewide voting power while dispersing others in districts in numbers too small to be a majority.

Critics have said partisan gerrymandering, when taken to extremes, warps democracy by intentionally diluting the power of some voters and the electability candidates they support.

Gerrymandering is a practice dating back two centuries in the United States. But critics have said it is becoming more extreme with the use of precision computer modelling to guide the creation of district boundaries that maximize the clout of one party's voters at the expense of other voters.

BREAKING: SCOTUS rules that partisan gerrymandering cases are a political question, meaning that courts can't decide them. This allows gerrymandering to continue and flourish.<br><br>It's now in the hands of voters across the nation to hold legislators to account and demand fair maps. —@ACLU

The <a href="https://twitter.com/hashtag/SCOTUS?src=hash&ref_src=twsrc%5Etfw">#SCOTUS</a> apportionment decision is a win for the American people. <br><br>We want elected representatives who are accountable to the people making decisions on district lines, not unelected bureaucrats. —@Jim_Jordan

While the Supreme Court has ruled against gerrymandering intended to harm the electoral clout of racial minorities, it has never curbed gerrymandering carried out purely for partisan advantage.

Democrats have said partisan gerrymandering by Republicans in such states as Wisconsin and Pennsylvania helped President Donald Trump's party maintain control of the U.S. House and various state legislatures for years, although Democrats seized control of the House in last November's elections and made inroads in state legislatures.

Presidential candidate Vermont Senator Bernie Sanders blasted the ruling on Twitter.

"Today, the Supreme Court gave Republican politicians across the country the approval to rig our democracy and suppress voters with racist, gerrymandered maps," said Sanders. "This is not what democracy is about."

"It's now in the hands of voters across the nation to hold legislators to account and demand fair maps," the American Civil Liberties Union said in a social media post.

But Ohio congressman Jim Jordan, a Republican, praised the decision.

"We want elected representatives who are accountable to the people making decisions on district lines, not unelected bureaucrats," said Jordan.

The cases were the last of the current term, which began in October.