Even if just the House flips, the Republican legislative programme and Trump's appointments would be stymied.
If Republicans hold Congress, they could continue their agenda and have a chance to tip the Supreme Court in their favour. Trump would get a springboard into 2020.
Trump has stamped himself on his party and will be front and centre of the Republican campaign for good or ill.
A key unknown is how much has the electorate changed since the 2016 election campaign, with all that has happened since.
On the Democratic side there have been signs of new enthusiasm over the past year and a half. Impressive special election wins, including in states won easily by Trump; the women's march and the anti-gun violence marches; protests over immigration.
New York Times analyst Nate Cohn tweeted: "Over the 1000+ special/general House elections in Dem-held seats in the Obama era, Republicans ran >20 pts ahead of district partisanship in a Dem-held seat just 4 times. Democrats have pulled it off 3 times in 7 shots in '17/18."
Trump has given the opposition plenty to oppose while stoking liberal anger in order to fire up his own followers.
Compare that to November 2016 when, after two terms of a Democratic government, Hillary Clinton was effectively asking for a third and was expected to win.
Despite the historic nature of her run as the first woman nominated, charisma and inspiration was at a low ebb.
The Democrats rely heavily on non-white voters, yet black turnout was down. Young voters, a key part of the Obama coalition, largely stayed away.
The challenge for Democrats is getting the balance right between taking on Trump, talking about issues of concern to voters and presenting new policies. In an arena where Trump sucks all the oxygen, that's not easy. An election dominated by Trump's outrageousness worked for him in the end in 2016.
Democrats must turn out their core base, be attractive to independents and woo some of the white working class voters in swing states who shifted from Obama to Trump.
Trump has been hitting familiar themes on immigration, the media, Washington, trade and gun rights at rallies and other speeches in an attempt to renew those bonds with supporters crucial to his win in 2016.
Amy Walter, of the Cook Political Report, tweets that "is why, I think, his approval ratings among Rs has risen in last 2 months. Q now is if Rs can match D intensity & if indies continue to spur on Trump/Rs".
Independents are now the biggest voting bloc, at 42 per cent, according to Gallup. A March NBC News/Wall Street Journal poll found that Democrats had a 12-point lead among independents.
Trump's strongest support comes from working class whites who make up more than 40 per cent of the electorate. A FiveThirtyEight.com report on a study by Washington think-tanks says that 23 states are 80 per cent white - illustrating Republican strength under the Electoral College system.
A political journalist for CNBC, John Harwood, quoting poll data, wrote at the weekend that: "Most Americans have considered Trump dishonest throughout his time in office. They judge his character indecent. But that no longer drives change in their judgments of his presidency."
He adds: "Americans have grown accustomed to Trump in the White House. The longer he serves without economic downturn or war, the more inured they become to his behaviour."
Harwood quotes David Winston, a Republican pollster: "They [Trump voters] knew the foibles. What they were trying to do was shake up the system. They have yet to reach a conclusion on: is it working, or not?"
In this stalemate of attitudes, events in the next six months, activism and voter turnout will be key.
According to preliminary figures, the US added 164,000 jobs in April and the unemployment rate dropped to 3.9 per cent. The New York Times said it was the 91st consecutive month of job gains.
A well-performing economy should be a powerful weapon for an incumbent president, though Trump's trade protectionism moves are still playing out.
Trump appears determined to turn his upcoming Korean summit into a historic symbolic triumph, while behind the scenes even throwing the complex issue of whether US troops should stay on the peninsula into the policy mix.
Experts worry about potential traps such as what happens should this high-risk diplomacy fail, what North Korea means by 'denuclerisation' and what impact the endangered Iran deal could have in discouraging Pyongyang the Trump Administration would stick to any agreement.
But Trump will see it as a major event to turn to his advantage in the midst of his domestic woes. He is in his element with symbolism and news distraction.
It will make him a dangerous proposition as an incumbent seeking re-election, should that happen.
WHAT THE KOREAN SUMMIT SHOWED
The Korean developments show how little has changed since the 2016 election in the way major events are followed by the public and how that suits Trump.
At the North and South Korean summit between leaders Kim Jong Un and Moon Jae In, there was a handshake across the demarcation line and small symbolic steps, hand in hand. There was the feel-good, vague reassurance in the summit communique that 'denuclerisation' was a future goal.
It illustrated the power of clearly defined and easily digested politics today. Nuance is a much harder sell with our churning news cycles and attention spans on stopwatch. And then there's the increasing way we absorb it all visually.
The situation was ripe for positive symbolism to be effective. Last year the world was rattled by rattling sabres and actual missile testing. Having had our toes singed with fiery rhetoric and fears, we responded well to a cooling balm.
To most people tuning in, what would have stayed with them would be the striking optics and the headlines of historic goodwill which we all wanted to hear.
Accepting the - hopeful and optimistic - broad brushstrokes of information and impressions from the summit would have been easy. People following the event would have had to make more of an effort to uncover reasons for scepticism and then consider them.
Logically, why would Kim, having spent billions on his nuclear programme, give it up when it has provided his new clout on the world stage.
Former North Korean deputy ambassador to the United Kingdom, Thae Yong Ho, told Radio Free Asia that the North will never agree to end its weapons programme irreversibly and with verification.
Pyongyang will likely seek a way to maintain its current nuclear status, while "denuclearising the Korean Peninsula," according to its own definition.
"The North has always wanted three things: First, the US to withdraw its nuclear weapons from the Korean Peninsula ... Second, to stop the US from deploying nuclear weapons or strategic assets to the peninsula and the region... Third, that the US announce it will not use nuclear weapons on the peninsula."
Thae said the summit with the US is North Korea's chance to show the world that it is a "normal nation" and a "big propaganda coup".
Trump's goal, also, will be to present the summit as a symbolic win and something he has achieved that his predecessors couldn't. And that's likely what most people will take from it.
BLUE WAVE OR RED WALL?
There are numerous signs that a blue Democrat wall could breach Congress, swamping the red Republican House of Representatives and possibly the Senate.
In favour
Special elections last year and this year, covering diverse districts across the country, have overall shown the Democrats with a +17 vote swing.
It's unlikely to be that huge an advantage in November - anti-Trump Democrats having been more inspired to turn out for special elections this time than Republicans. The bombshell Democratic wins in Alabama and Pennsylvania featured turnout similar to that of Midterm elections.
But another indicator is that Democrats lead the generic party ballot and have done consistently for the past year. RealClearPolitics.com yesterday put them ahead by 45.6 per cent to 39.4 per cent, FiveThirtyEight.com by 47.2 per cent to 40.2 per cent.
Vox points out that when one side has a large lead there's usually a big result: "2006 Democrats +11.5, wave flips House to Democrats; 2008 Democrats +9, wave further increases Democratic House majority; 2010 Republicans +9.4, wave flips House to Republicans."
Not in favour
Polling data shows the general climate is also difficult for Republicans. When pollsters ask about the direction of the country, those who think it is on the wrong track outnumber by 55.6 to 35.4 per cent those with a positive view, RCP reports.
Trump's approval has been slowly ticking upwards from a low of 37 per cent in the RCP average last December to 44 per cent yesterday. The majority - 52.6 per cent - still disapprove. FiveThirtyEight yesterday put Trump's approval at 41.4 per cent and his disapproval at 52.5 per cent.
Retirements
The high number of Republicans calling it quits - 41 in Congress altogether including House Speaker Paul Ryan - is seen as a recognition this is not their year. That compares with 18 House Democrats retiring.
The House
The key number is 218 for a ruling majority. All 435 seats will be contested and the Democrats need to pick up 23 Republican seats. RCP rates 31 seats as toss ups.
The Senate
Fifty one seats are needed for a ruling majority. There are 35 seats up for grabs this year and only nine are held by Republicans. The Democrats need to gain two of the nine and hold on to their own. Only one of the nine seats is in a state won by Democrat Hillary Clinton in 2016 and 10 Democrats face re-election in states won by Trump. RCP rates eight seats as toss ups.
Governors
There are governorship races in 36 states and three territories. The mayor of the District of Columbia is also up for election.
- Nicola Lamb is the Foreign Editor of the NZ Herald
