With the tipped "blue wave" only hitting the House, Republicans are on course to increase seats in Senate
Why Democrats had a struggle to take both houses
Despite some worries that low approval ratings of US President Donald Trump would cost Republicans control of both chambers of Congress, Democrats were facing an uphill battle.
With vote counting continuing, Republicans look set to keep and even increase their slim Senate majority as they lose grip on the lower House of Representatives.
The result is broadly in line with what many analysts projected but makes for a confusing picture.
So how are the Democrats on course to soundly rout the Republicans in the House and lose seats to them in the upper chamber?
To put it simply, different rules govern elections for the two chambers.
The 100 members making up the US Senate serve six-year terms and the elections for those seats are staggered. This means that only one third of those seats are up for re-election during mid-terms.
So why did the Democrats lose these seats? Because they were defending more vulnerable Senate positions this time. Some of those seats, won back in 2012 when former president Barack Obama was ending his first term, were in particularly conservative parts of the country such as Indiana and North Dakota. In these typically “red states”, Democratic incumbents lost their re-election bids.
But for the House of Representatives, consisting of 435 members, all of those seats are generally up for grabs every two years. Those numbers were significantly more favourable to Democrats, enabling them to make gains.
Peter Yacobucci, an associate professor of political science at Buffalo State University, explained the origin of the rules.
“United States voting rules are an artefact of the creation of the country,” Mr Yacobucci said, referring to Article One of the US Constitution, which established the House of Representatives and the Senate.
The rules and mixture of term lengths were designed as a system of checks and balances, ensuring equitable representation among different populations in the US and for elections to take place at different times to smooth out the bumps and troughs of the prevailing political mood of the day.
There was some hope among Democrats that an energised electorate angry at President Donald Trump would cause the party to beat expectations and take both legislative chambers. Many spun it as a referendum on his two years in office.
But Mr Yacobucci said that, even with an increase in Democratic turnout, midterm voters in general tend to skew towards the conservative, somewhat blunting the impact.
“Traditionally midterm elections are dominated by those in the upper-income brackets and the elderly,” he said, pointing out that the wealthy tend to vote at more than double the rate of the poor.
Mr Yacobucci said there were fewer independent “swing voters” in recent years, making it more difficult for Democrats this time.
While the remaining states are still working their way through boxes of ballot papers and the final tally is still to be called, broadly speaking the overall result looks clear and Mr Trump's next two years will look very different from the past.