America is hurtling towards the first nationwide test of its democracy since Donald Trump left the White House and his supporters mounted a deadly insurrection at the US Capitol. The midterm elections on Tuesday will decide control of Congress as well as 36 state governorships – but will also be a referendum on Joe Biden’s presidency, and fire the starting gun for the race to the White House in 2024.

Here’s why the midterms matter for some of the biggest issues facing the US – and the world:

The 2024 presidential election

A frenzy of speculation over the 2024 presidential race is likely to begin even before the last vote is cast in 2022. Midterm elections are typically seen as a referendum on the incumbent president. If Democrats suffer heavy losses in the House of Representatives, Senate and state governors’ mansions, the buck stops with Biden.

There may be calls, especially from the left, for him to announce that he is not running again. He turns 80 on 20 November and is already the oldest president in American history. His potential successor would need time to build a political brand and establish a fundraising apparatus. But there is no obvious heir apparent and Biden can point to history: both Bill Clinton and Obama suffered midterm rebukes in 1994 and 2010 only to bounce back and win reelection.

On the Republican side, Trump faces a test of his own electoral viability. A pattern of defeats for candidates he endorsed in states such as Arizona, Georgia, Pennsylvania and Ohio might prompt more pragmatic Republicans to question whether the 76-year-old former president represents the party’s best shot at the White House. But victories for candidates who support Trump’s big lie about a stolen election would also raise fears about democracy itself in 2024. David Smith

Ukraine

Russia’s invasion of Ukraine in February 2022 shocked Americans. Eight months later, most Americans continue to support efforts to aid Ukraine though polling suggests that the war – and foreign policy more broadly – isn’t one of the major issues driving their vote this cycle.

Nevertheless, the midterms could have dramatic consequences for Ukraine. Republican leader Kevin McCarthy, who hopes to become the House Speaker if his party wins control of the chamber in November, has said Congress would no longer “write a blank check to Ukraine”. Those comments drew strong rebukes from Democrats and divided Republicans.

Biden has also repeatedly spoken about the war in the context of rising fuel prices, a top concern among voters that Republicans have used to attack Democrats’ economic policies. In response, Biden has sought to blame Russian president Vladimir Putin, saying high gas prices are the cost of imposing crippling sanctions on Russia for invading Ukraine. The US president has also argued that standing with Ukraine is critical to defending democratic values, which are under attack around the world.

On the campaign trail, a growing number of Republicans have criticized Biden for sending aid to Ukraine while Americans are suffering from high inflation at home, a sign of a wider fissure in the party over the war. In some instances, far-right acolytes of Donald Trump, like Georgia congresswoman Marjorie Taylor Greene, have even echoed Moscow talking points. Such views are only a minority in the party, but if Republicans win the House, members like Greene will surely see their political clout rise. Lauren Gambino

Democracy

Several Republican candidates who have doubted the 2020 election are on the cusp of winning gubernatorial, attorneys general, and secretary of state contests in Arizona, Wisconsin, Nevada, and other key battleground states. Those offices all play a role in ensuring that votes are lawfully cast and counted. If these candidates win, they would oversee the 2024 presidential election in their states. There is already deep concern they would use their positions to sow confusion to try to overturn the result of the 2024 vote, refusing to seat any candidate who defeats Trump.

Jim Marchant, a Republican running for Nevada’s top election official, has said explicitly this is his goal: “When I’m secretary of state of Nevada, we are going to fix it, and when my coalition of secretary of state candidates around the country get elected we’re going to fix the whole country, and President Trump is going to be president again in 2024,” he said at a rally last month.

Meanwhile, if Republicans take control of the US House, as they are expected to, election denialism will be prominent among its members there too: 124 candidates who either denied or doubted the election are heavily favored to win their contests, according to FiveThirtyEight. Republicans could use their new majority to launch investigations, hold hearings, and spread misinformation about elections.

A Republican victory in either House of Congress would quickly end whatever slim chance remained of Democrats passing any kind of federal voting rights legislation. That stalemate would essentially preserve the status quo for at least another two years. Even though they have controlled both chambers of Congress since 2020, Democrats have been unable to pass any kind of voting rights legislation because of the filibuster, a procedural rule in the US senate that requires 60 votes to advance legislation. Sam Levine

Judges

Trump appointed more than 200 judges to the federal judicial system during his single four-year term – arguably his most profound legacy. The judges’ decisions touch millions of lives. This was never better illustrated than in June when the supreme court – with three out of nine justices appointed by Trump – overturned the constitutional right to abortion.

Biden has been fighting back. As of 8 August, he had appointed 75 judges to the federal bench, far more than Trump or Barack Obama at the same stages of their presidencies. These included a record number of women and people of color, most notably Ketanji Brown Jackson, the first African American woman to serve as a supreme court justice.

All this has been made possible by a Senate under Democratic control. Should Republicans win the chamber, they will have the power to block Biden’s future nominations, likely forcing him to choose “moderate” candidates in the hope of picking up Republican votes. Few Democrats have forgotten how Senate Republicans froze a supreme court vacancy in the last year of Obama’s presidency and denied Merrick Garland a hearing. David Smith

Investigations and committees

The Biden administration and Democratic lawmakers are bracing for a legislative blockade and an onslaught of investigations. If Republicans take the House majority, one of the first orders of business will likely be terminating the work of the select committee investigating the January 6 insurrection. Members of the committee have anticipated this possibility as well, and they are racing to release a full report of their findings before the end of the year. Rodney Davis, the Republican ranking member of the House administration committee, has even indicated plans to investigate the work of the select committee, which has consistently attracted the ire of Donald Trump since its creation last year.

Such an investigation would just be one of many investigations launched by House Republicans if they take control of the lower chamber. Republican leaders have suggested they are looking to investigate the overseas business dealings of Hunter Biden, the president’s son, and the US troop withdrawal from Afghanistan last year, among other topics.

House Republicans’ agenda may even include the impeachment of a sitting cabinet member. Multiple members of the Republican caucus have called for the removal of Alejandro Mayorkas, Biden’s homeland security secretary, over the White House’s handling of the US-Mexican border. “We will give Secretary Mayorkas a reserved parking spot, he will be testifying so much about this,” House minority whip Steve Scalise said in September.Joan E Greve

Abortion

The supreme court’s decision to overturn Roe v Wade in June catapulted the issue of abortion to the front and center of several key midterm contests. The outcomes of governor’s races in particular could have direct consequences on the future of abortion access, which will now be decided by the states.

Democrats have made abortion a core part of their campaign message amid signs that fury over the ruling – and over Republican-led efforts to ban abortion in the states – was fueling a political backlash. In several states, the number of women registering to vote surged and in conservative Kansas, voters overwhelmingly rejected an attempt to undermine abortion protections.

Abortion protections are on the ballot in four states, while competitive contests for state legislature and the governor’s mansion could be critical to determining access in the state. At the federal level, Biden has vowed that his first legislative act of the new Congress would be to codify abortion rights, if Americans deliver Democrats even bigger majorities this November. By contrast, some Republicans have said they would push for a national ban on abortion if their party retakes control of Congress in November. Lauren Gambino