Donald Trump has voiced confidence about the upcoming midterm elections, where his name is not on the ballot but which many see as a referendum on his presidency so far.

“I have the same feeling that I had in 2016,” he recently told USA Today as he flew to Texas to campaign with Ted Cruz. “I think we’re going to do well.”

The paper said he had repeatedly expressed optimism Republicans would not only hold on to their majority in the Senate but perhaps even expand it, by bagging seats in places such as Montana, Nevada, North Dakota or and Indiana. He will have held 30 campaign rallies and fund-raising events by November 6.

Asked if the elections were a referendum on his presidency, he replied: “No, but I think I’m helping.”

But most observers believe the tide is against the president and his party. While 2018 might not see the “blue wave” some had predicted recently, most believe the Democrats will do enough to flip the 23 seats they need to win the House of Representatives.

The political analysis site Five Thirty Eight says there is a 6 out of 7 chance (85.9 per cent) the Dems will win it, and a 1 in 7 chance (14.1 per cent) Republicans will hold it.

At the same time, while most believe the Republicans will hold onto their slim majority in the Senate, there’s a chance Democrats could take that chamber too. Five Thirty Eight gives the Democrats a 2 out of 9 chance of wining the senate (21.9 per cent), and the Republicans a 7 in 9 (78.1 per cent) chance of holding on to it.

Other than Mr Trump having to eat crow if things go badly for Republicans this autumn, these are the most important likely consequences:

Losing the House:

The thing that Mr Trump and Republicans are most worried about is also the thing most likely to happen - the loss of the house. This is where the majority of legislation begins, and if Democrats took control, they would have considerable leverage in not only disrupting Mr Trump’s agenda, put pushing their own.

They would have to work with the senate, but the free-run the Republicans have enjoyed controlling the White House and both houses of congress, would be over.

Crucially, the house is where any impeachment proceedings against Mr Trump would begin. While Democrats have made a decision not to talk about this ahead of the election, if they are the majority when the new house sits in January they might feel very differently. Many of the newly elected, younger members would likely push for it - especially if Robert Mueller’s investigation comes up with something damaging.

US midterm elections: the campaign trail Show all 20 1 /20 US midterm elections: the campaign trail US midterm elections: the campaign trail President Trump speaks at a campaign rally in Missoula, Montana AFP/Getty US midterm elections: the campaign trail Vermont Senator Bernie Sanders endorses Democrat Congressional candidate Randy Bryce at a campaign rally in Kenosha, Wisconsin EPA US midterm elections: the campaign trail Oprah Winfrey interviews gubernatorial candidate Stacey Abrams in Marietta, Georgia Reuters US midterm elections: the campaign trail Democrat US Senator Bob Menendez stands with representatives from local police and fire departments during a campaign event in Jersey City, New Jersey EPA US midterm elections: the campaign trail Democrat candidate for Senate Beto O'Rourke addresses supporters in Carrollton, Texas Reuters US midterm elections: the campaign trail Democratic congressional candidate Angie Craig and US Representative Tim Walz at a campaign event in Mendota Heights, Minnesota AFP/Getty US midterm elections: the campaign trail Protesters of President Trump outside a campaign rally in Rochester Minnesota AFP/Getty US midterm elections: the campaign trail Supporters of President Trump outside a campaign rally in Rochester Minnesota AFP/Getty US midterm elections: the campaign trail Florida Republican gubernatorial candidate Ron DeSantis, left, shakes hands with Democratic gubernatorial candidate Andrew Gillum after a debate on CNN AP US midterm elections: the campaign trail President Trump takes the stage at a rally in Elko, Nevada Reuters US midterm elections: the campaign trail President Trump addresses a rally in Elko, Nevada AFP/Getty US midterm elections: the campaign trail Donald Trump T-shirts for sale at a rally in Rochester, Minnesota AFP/Getty US midterm elections: the campaign trail John Lombardo talks with voters on the doorstep in Wilkes Barre, Pennsylvania AFP/Getty US midterm elections: the campaign trail Democrat Senator Joe Manchin hugs supporters as he marches in a homecoming parade in West Virginia AFP/Getty US midterm elections: the campaign trail A supporter holds up a sign at a Trump rally in Chattanooga, Tennessee EPA US midterm elections: the campaign trail Supporters of Republican congressional candidate Pete Stauber attend a campaign event in Crosby, Minnesota AFP/Getty US midterm elections: the campaign trail Former President Barack Obama joins Democrat Congresswoman Jacky Rosen on stage in Las Vegas at a rally for her Senate candidacy EPA US midterm elections: the campaign trail Members of the Culinary Union prepare packets for canvassing for the Democrats in Las Vegas AP US midterm elections: the campaign trail Republican volunteer Jess Morgan, 69, campaigns for her party in Shavertown, Pennsylvania AFP/Getty US midterm elections: the campaign trail Native American candidate Deb Haaland who is running for Congress in New Mexico AFP/Getty

Losing the senate:

If the Republicans had to lose one of the chambers, they would probably opt for the senate. But even if they clung onto the house and lost the upper chamber, Democratic senators could cause all manner of problems for the president by calling hearings into his business interests, his apparent conflicts of interests and just about anything they wanted to. Dianne Feinstein, the senior Democrat on the Senate Judiciary Committee, recently said they would hold a new investigation into Supreme Court justice Brett Kavanaugh, who was accused of sexual assault by at least three women, should Democrats take control. The judge denied the allegations. “Oh, I’d be in favour of opening up the allegations, absolutely,” she said.

Magic touch disappears:

Mr Trump has long bragged of his ability to help Republican candidates he endorses with the backing of himself and the White House. In a number of cases, he has been correct. Karen Handel of Georgia, Ron Estes of Kansas, and Greg Gianforte of Montana all won special elections against Democrats last year after receiving his support. Others who earned his backing - Roy Moore in Alabama, Rick Saccone in Pennsylvania and Ed Gillespie, all lost. Mr Trump has so far endorsed 83 candidates since taking office. If most of those contesting the midterms, he will claim to have retained his magic trust, even if the party loses the senate or house. If most of his candidates lose, it will be a humiliation.

Challenge from within:

While it is the Republican leadership of the senate and house who will bear most responsibility should they lose one of the chambers, Mr Trump will not be able to escape sharing some of the blame - especially given how hard he has campaigned for a win.