What happens after the US election on November 8 will be shaped not just by who wins, but by the president's relationship with the two houses of Congress. As usual, one third of the Senate and all of the House of Representatives are up for election on the same day as the president. Both are currently under Republican control - but some Democrats are starting to wonder if they might just take both of them back.
Contests for control of the Senate have typically paralleled presidential races. Historically, the makeup of the House of Representatives has also generally matched the outcome, with the party that wins the presidency doing better at the congressional level, too.
If anything, this relationship has only become stronger in recent election cycles. A Trump victory is highly likely to bring with it Republican majorities in both the Senate and the House, and in a typical year, we would expect a victorious Hillary Clinton to also carry a fair number of House Democrats into office with her. But even if she wins, can the Democrats really retake the House?
One of the things that makes this year's election cycle so unusual is the possibility that some voters could choose Republican candidates for House seats while voting against Donald Trump, who is much more distant from the mainstream of his party than most presidential candidates. Trump won the presidential nomination against considerable establishment opposition, and he's still not on friendly terms with the party elite. Some Republican house candidates haven't endorsed him, and some are even explicitly distancing themselves from him.
This means that many more voters than usual might think separately about their congressional and presidential choices - and if that link is broken, it means presidential votes pulled from Trump to Clinton won't necessarily bring congressional votes with them.
Then there's the risk that voters will go to the polls expecting Hillary Clinton to win. Notwithstanding recent developments in the saga of her State Department-era emails, she remains favoured by electoral projections. If this perception remains, that could actually undermine house Democrats' chances. Recent research has shown that, historically, house Democrats have done worse at the polls in years when their presidential candidate was considered highly likely to win. This may indicate that voters like to "balance" their choices in the hope that the presidency and the two chambers of Congress aren't controlled by one party.
Both of these reasons are quite specific to this election year. But there's another obstacle in the Democrats' way - one that they can't do much about.
Even if Hillary Clinton wins a victory that helps her Congressional colleagues win more votes, the potential for Democrats to obtain a majority of seats in the House is nevertheless quite unlikely due to the current structure of legislative redistricting.
The legislative redistricting process is heavily influenced by partisan "gerrymandering." Legislative redistricting is the process of adjusting constituency boundaries, which should contain an equal number of residents, to reflect population changes. Gerrymandering - named for a salamander-shaped district designed by one of redistricting's earliest innovators - refers to the practice of using this process to create districts, often with torturous shapes, to provide an advantage to certain politicians.
In many states, redistricting is controlled by the state legislature, meaning that whichever party controls the legislature can optimise the boundaries of their state's House districts along demographic lines. When this happens, the result is to dilute the concentration of voters likely to vote for the opposing party.
This is done by drawing boundaries that divide the supporters of a party so that this party will win majorities in the fewest possible districts. When partisan gerrymandering is successful, it's remarkably effective: it can mean a party that wins 50% or more of the statewide congressional vote only wins 40% or fewer of the state's congressional seats.
Gerrymandering has long been used by both parties, but the current situation clearly favours the Republicans. The last round of redistricting was held just after Republicans took over a remarkable number of state legislatures in 2010. It also came after a 2004 Supreme Court ruling which decided that partisan gerrymandering need not be subject to judicial oversight - meaning that new district boundaries can no longer be legally challenged on grounds of partisan bias.
The results are plain to see. According to recent research, the partisan bias that's emerged since 2010 is even more marked than it was in previous decades. The upshot is that Democrats now need at least 54-55% of the national congressional vote to win control of the House. Sure enough, President Obama's comfortable re-election in 2012 didn't translate into significant Democratic gains in the House, even though Democrats won a larger overall share of the nationwide congressional vote.
Take all these obstacles together and Clinton almost certainly needs to win by an extraordinary margin to secure an allied House of Representatives.
While it is by no means clear what Trump's relationship with a Republican-controlled Congress would look like, the recent history of divided government certainly suggests there is cause for concern.
US politics in recent years have been particularly polarised and acrimonious. The resulting gridlock has destabilised federal policy-making at a time when the nation faces severe challenges. The raw anger on display in the 2016 presidential election is not going away - and another two to four years of deadlocked government will hardly help.