The November 6 midterm elections could pose serious problems for US President Donald Trump, if Democrats take back the House of Representatives. Losing the House means investigations will open into everything, from his tax returns to alleged affairs, and assault allegations against Justice Brett Kavanaugh. It could also lead to Trump’s impeachment over ties between his election campaign and Russia, possible obstruction of justice and block further appointments to the Supreme Court.

Experts say Donald Trump would also find it exceedingly difficult to push through the kinds of legislation that he and his administration have so far been favouring during the first half of his presidency, if his party loses. All foreign policy leadership positions in Congress will also change after the midterms. While foreign policy as an individual issue will likely not be a key election issue, the outcome of the Congressional election could impact the Trump administration’s conduct of foreign affairs in various ways. Should Democrats wrest the Senate from Republican control, which is currently deemed unlikely but not impossible, it would give them significant sway over foreign policy. Just as a Democratic takeover of one or both chambers of Congress would allow them to slow down or block Trump on foreign policy, they could do the same on domestic policy — but much more forcefully. That’s because the constitution allows Congress a lot more input into domestic policy than into foreign policy, which is generally the remit of the executive branch.

Political observers believe Democrats are going into the final stage of the campaign in a strong position. According to the Financial Times, the midterm elections are shaping up to be the most consequential in a generation. The stakes are high for the Republican party, which controls both chambers of Congress but risks a possible rout in the House of Representatives due to a large number of retiring members and anti-Trump anger energising Democrats. Charlie Cook of the Cook Political Report notes that the party that controls the White House has lost House seats in 35 of the 38 midterms since the civil war. The Democrats need a net gain of 23 seats to take control of the House. The report estimates that 69 of the 435 races are competitive — where both parties have a realistic chance of winning — while another 39 races have the potential to become competitive. Of the 31 seats in the House that it rates as a “toss-up”, 29 are currently held by the Republicans, while another 12 of the competitive seats are rated “lean Democratic”. Facing a potential Democratic wave, Republicans are taking solace in the buoyant economy, with unemployment now down to 3.7 per cent. On the other hand, Democrats are confident that they will win the House, even with the economy doing well. While they are campaigning on bread-and-butter issues such as healthcare and calls to reverse the recent tax cuts, the midterms are generally seen as a way to score the first two years of a president’s time in office.

Democrats are optimistic about taking back the House. When a president has an approval rating above 50 per cent, the historical record suggests that the midterm losses for his party can be limited. Trump’s rating has been hovering in the low 40s. Some experts say the elections will actually be a referendum on Trump. “It is a referendum on Trump for the most part,” says Howard Dean, a Democratic presidential contender in 2004. “I expect to win the House and I actually think there’s a decent chance we will win the Senate.” Another driver of Democratic optimism is the mood among women voters. The party has fielded a record number of female candidates. Many of the candidates are running in competitive districts, and are being helped by strong fundraising campaigns.

The elections will also determine the future course of action for the US and the world. Foreign policy played an outsize role in the presidential race that Donald Trump won in 2016 and could be a factor again in the elections on November 6 — albeit in an indirect way. “What the midterms usually are is a reflection of presidential popularity. Trump’s foreign policy efforts have been pretty unpopular on average with the American public — that’s what most of the recent polling shows — and that had a depressing effect on his popularity,” Trevor Thrall, a foreign policy scholar at George Mason University and the Cato Institute, told the Deutsche Welle.

A probe into the Trump administration’s crackdown on undocumented immigrants could be one consequence of a Democratic-led House. Another would surely be the ramping up of existing investigations and the possible launch of fresh probes into the Trump campaign’s connections with Russia. None of this would change the direction of Trump’s foreign policy. But all of it would make it a lot more difficult and time-consuming for Trump to carry out his policy, which could lead him to assess whether advancing any specific foreign policy goal is worth the fight and the political price he might have to pay to push it through Democratic roadblocks in Congress. Just as a Democratic takeover of one or both chambers of Congress would allow them to slow down or block Trump on foreign policy, they could do the same on domestic policy — but much more forcefully. A Democratic House would spell the end to any Republican hopes of fully eliminating Barack Obama’s healthcare reform.

Democratic women candidates are also being propelled by the #MeToo movement. Many are angry at how the Republicans handled the claims of assault against Brett Kavanaugh, now a Supreme Court judge. Another crucial factor in the election will be whether young people vote in large numbers or not. While many Republicans are pessimistic about their odds in the House, they are much more confident about retaining control of the Senate. Democrats face a very tough electoral map since 26 of the 35 senators up for re-election are from their party, and 10 of those are in states Trump won in 2016. Republicans have a 51-49 majority in the Senate, so the Democrats need a net gain of two seats to win. While they have several narrow paths, they would have to hold their five most vulnerable seats and win two of the four “toss-up” Republican seats — Arizona, Nevada, Tennessee and Texas — or a similar combination. The Kavanaugh battle may motivate Democrat voters in House races but it could hurt their Senate chances, where the party is defending several seats in conservative states.

If Democrats wrest the Senate from Republican control, which is currently deemed unlikely but not impossible, it would give them significant sway over foreign policy. Since Senate approval is required for executive branch nominations and international treaties, Democrats could block the appointment of foreign policy officials and put the breaks on trade agreements, a cornerstone of Trump’s political agenda, and other international accords. That’s why for President Trump and Congressional Republicans losing the Senate is a nightmare scenario: It would give Democrats a strong tool to push back against the White House’s so far fairly unconstrained foreign policy machinations. But even if the Democrats win back the House of Representatives from the Republicans, which is deemed likely but not certain, they could influence Trump’s foreign policy, although to a much lesser extent than if they captured the Senate.