Their goal is to make life difficult for Donald Trump. Midterm wins could even lead to impeachment proceedings
Can Democrats take back the House in November?
It took deep pockets and some words of persuasion, but Democratic Party leaders are confident they are moving toward a decisive victory in November’s midterm elections after putting up a united front in the past week’s primaries.
For the most part, mainstream candidates saw off fringe runners, and they dodged pitfalls in California – America's most populous state – where the voting system and infighting could have split the Democratic vote.
But it is unlikely to be a walkover: Republicans also claimed a solid set of results, particularly for those from the wing of the party closest to Donald Trump.
The voting also suggests the midterms are shaping up to be an election that pits candidates identifying with the US president against centre-Left runners.
Reed Galen, an independent political strategist who saw the campaign in California, said Democrats would be encouraged.
"Democratic primary voter participation has generally gone up, I think, pretty significantly and where you see that you are getting a more representative cross-section of what the party is, which is not necessarily so far left as Bernie Sanders," he said.
The trend was on display in New Jersey, where several establishment-backed Democrats saw off more-liberal challengers in Tuesday’s primaries.
In California it meant a primary win for Gil Cisneros, a Navy veteran and former Republican who was favoured by the party establishment over the likes of Andy Thorburn, who campaigned on the Sanders-esque platform of universal healthcare.
Their bad-tempered rivalry illustrated another feature of the race in a state where the top two vote winners move from the primaries to the election, irrespective of party. With four runners in the district, Democratic party leaders feared that bad blood could split the vote and allow two Republicans to go forward.
So they poured at least $2-million into the Cisneros campaign and ordered the pair to meet at an Italian restaurant to settle their differences. Similar moves were made in other districts, where some candidates were encouraged to switch seats to spread the vote.
Such are the stakes. Democrats need to flip 23 seats, nationally, to take control of the House of Representatives.
Not only would that help them sabotage Mr Trump's policy agenda, they could even use a majority to impeach the president.
Historical precedent suggests they have reasonable a chance of success. Although Mr Trump can count on a booming economy and record low unemployment, the party that holds the White House can expect to lose seats in Congress.
California is crucial to the eventual outcome in November. Democrats have set their sights on 10 of the 14 House seats held by Republicans in the state.
Although the results across California will take weeks to count, high turnouts suggest that Democrats are competitive in seven of the Republican districts won by Hillary Clinton in 2016.
Tom Perez, chair of the Democratic National Committee, said in an email to supporters: "These primary results proved Democrats have momentum and we are harnessing the growing enthusiasm across the country – and together we came out of last night with some incredible wins."
But they will not have it all their own way, even in a liberal state such as California.
Strong turnouts by Republican voters mean Democrats will face tough challenges in conservative territory they have targeted in suburban areas near Los Angeles and Sacramento.
If Democrats show signs of fighting the election from a more centrist position, the same cannot be said of Republicans.
"From Indiana to Arizona to Ohio, the name of the game for Republican candidates this primary cycle has been to flaunt their Trump love," said The New York Times editorial board. "And woe unto anyone deemed insufficiently smitten."
Martha Roby, a four-term Republican Representative, was deemed "insufficiently smitten" by the voters of Alabama. She was the first member of Congress to withdraw her endorsement of the then Republican candidate in 2016 when a recording emerged of Mr Trump bragging about his womanising behaviour.
"I cannot look my children in the eye and justify a vote for a man who promotes and boasts about sexually assaulting women," she said at the time.
During the campaign she emphasised her subsequent working relationship with the White House, but it was not enough to secure the 50 per cent of the vote she needed. She now faces a run-off against a challenger next month.
Mr Trump could also claim victory in backing John Cox who secured a spot on the ballot for California governor. Although he has little chance of winning, Republicans feared they might end up without a candidate for the most important job in the state.
"Even Fake News CNN said the Trump impact was really big, much bigger than they ever thought possible," wrote a triumphant Mr Trump on Twitter. "So much for the big Blue Wave, it may be a big Red Wave."
Not according to Mr Galen and the majority of polls, which point more towards a small blue wave.
"If they... the Democrats... take the house it won't be by an overwhelming number," he said.
It is perhaps no surprise that the midterms are shaping up to be a referendum on Mr Trump’s time in office. But last week’s primaries are starting to chart the way that will play out before November.