Virtually every politically relevant indicator points to the Democrats winning a majority of seats in the US House of Representatives at next week's midterm elections.

Public opinion polls have long suggested an 8-to-9-point lead for a generic Democratic House candidate over a Republican candidate. President Trump's approval ratings, in the low 40s, are historically associated with about a 40-seat loss for Republicans, more than enough for Democrats to win a majority.

Far more Democrats participated in primary elections than did Republicans, a signal of Democratic enthusiasm exceeding that of Republicans. More Republican incumbents retired ahead of the 2018 elections than did Democrats, indicative of a good electoral environment for Democrats.

But partisan gerrymandering — the practice of drawing electoral boundaries that favour one party over another — could be an ace up the Republican sleeve in next Tuesday's elections.

Stacking the deck

Elections for Australia's federal parliament are administered by the Australian Electoral Commission, a non-partisan, professional agency, charged with maintaining the electoral roll, the conduct of elections and conducting electoral redistributions, dividing states and territories into seats for Australia's House of Representatives.

In recent decades, the determination of electoral boundaries appears to be free of partisan interference and manipulation.

The situation is different in the US, where state and local governments have responsibility for the administration of elections, including electoral redistricting. The US Constitution provides that every 10 years a census shall be conducted to determine the apportionment of the House of Representatives seats (Congressional districts) across the 50 states, in proportion to the population of those states.

The US Supreme Court insists on strict adherence to equal population in each district, ruling out an older form of electoral manipulation known as malapportionment.

In at least 26 US states, redistricting is a political affair, an act of the state legislature, requiring majority approval from the houses of the state legislature and the governor's assent. In 2011-12 — the last redistricting — Republicans controlled redistricting in 17 of these 26 states and Democrats in five states, with divided government prevailing in two of the 26 states. Courts or commissions controlled redistricting in 14 states.

Partisan gerrymandering arises when the party creating districts "packs" their opponents' voters into a relatively small number of districts and "cracks" their opponents' remaining voters such that they form minorities in the remaining districts.

Indeed, more extreme partisan gerrymanders can see the favoured party winning a majority of seats without winning a majority of votes.

Protesters wear cut-outs of congressional districts at a protest at the Supreme Court in March over alleged gerrymandering in Maryland. ( Reuters: Joshua Roberts )

Legal challenges to gerrymandering

In 2017 I testified for plaintiffs claiming that North Carolina's congressional districts constituted an unconstitutional, pro-Republican gerrymander. The three-judge Federal Court hearing the case agreed. The case is currently on appeal with the US Supreme Court.

The evidence I produced revealed several characteristics of partisan gerrymandering. First, although Democrats won 46.7 per cent of the House vote state-wide in 2016, they won only three out of 13 seats; conversely, Republicans won 10 out of 13 seats with 53.3 per cent of the vote, an unusually disproportionate result.

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Second, it is evident that the Democratic vote has been "packed" into those three districts where Democratic candidates win by margins of no less than 67-33. The remaining Democratic votes in North Carolina are dispersed throughout the 10 seats won by Republicans, with the most marginal Republican win was 56-44.

Third, the apparent "packing" and "cracking" of Democratic partisans is no mere peculiarity of the candidates and issues in North Carolina's congressional races in 2016, but is also closely mirrored in votes for Clinton and Trump in the presidential election.

North Carolina's districts were clearly designed with the partisanship of the voters in mind, as the Republicans who drafted this set of boundaries brazenly conceded.

North Carolina Democrats require a 6.1 percentage-point swing from 2016 results to pick up a fourth seat, which would take their state-wide vote share to 52.8 per cent. To win seven (a majority) of North Carolina's 13 CDs, the swing needed is 8.4 per cent, at which point Democrats would have 55.1 per cent of the state-wide vote. Conversely, Republicans can retain seven out of North Carolina's 13 seats with as little as 44.9 per cent of the vote.

It's not just North Carolina

Similar patterns hold in other seats with aggressive pro-Republican gerrymanders.

Republicans can retain a majority of Florida's 27 Congressional districts with far less than a majority of the state-wide congressional vote, just 44.6 per cent.

In Michigan, Democrats require a 6.9 per cent swing to pick up an additional seat; Republicans can retain a majority of Michigan's 14 Congressional districts with as a little as 43.7 per cent of the state-wide vote.

In Ohio (16 Congressional districts), Democrats need a 9.2 per cent swing for a fifth seat; Republicans can retain eight of Ohio's Congressional districts with as little as 42 per cent of the state-wide vote.

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Maryland is perhaps the clearest case of a pro-Democratic gerrymander at present. Republicans hold just one of Maryland's eight Congressional districts, winning it 70-30, consistent with Republican voters being packed. But with 52.2 per cent of the state-wide vote Republicans could win four seats there, a mild level of partisan bias relative to the large pro-Republican biases in Ohio, Michigan, Florida and North Carolina.

The midterms

With polls suggesting Democrats will outpoll Republicans by about 8 percentage points, one might reasonably conclude that Democrats will win a comfortable majority in the House.

They may. But gerrymandering systematically reduces the number of marginal Republican districts, meaning that Democrats must reach quite high "up the tree" to win the 23 or so seats they need to form a majority.

By design, partisan gerrymandering suppresses the responsiveness of elections, stifling the translation of more votes for one party into additional legislative seats for that party.

Nationally, Democrats could require more than 55 per cent of the two-party vote for Congress to win 50 per cent of the seats in the Congress.

Under a fair system, each party should have roughly the same chance of forming a majority if they win roughly half of the votes.

That is manifestly not the case in contemporary American elections. The systematic, deliberate manipulation of district lines — to suppress the responsiveness of legislatures, law and policy to public opinion — is a corruption of American democracy, and alas, could well play a key role in next Tuesday's elections.

Professor Simon Jackman is chief executive of the US Studies Centre at the University of Sydney.