US election: how control of Congress will matter for the new president

Kamala Harris and Donald Trump are promising big initiatives if elected: tax cuts (and hikes), lots of giveaways, and major pieces of legislation bearing on issues such as abortion, healthcare, the environment and foreign military assistance. Regardless of who wins the presidency, the one thing all these items have in common? They can’t pass without Congress, which comprises the House of Representatives (the lower body) and the Senate (the upper body).

The Senate is currently controlled by Democrats, 51 to 49, while Republicans hold a majority in the House of Representatives, 220 to 212. Website FiveThirtyEight, which aggregates polls, forecasts that the Republicans are far more likely to win the Senate 2024. In the House, the race is expected to be much closer.

Given the numbers, it’s the Senate that most worries Democrats and excites Republicans. Democrats are likely to lose representation in Republican-leaning West Virginia, and could lose additional seats in Ohio, Montana, Michigan, Pennsylvania and Wisconsin. There’s a chance for Democrats to pick up seats in Florida and Texas, but both races are still trending Republican.

Who wins the Senate could constrain the next president, if the party of opposition is in control. In the Senate, the filibuster, a tactic to delay or block legislation, can make it hard to enact many new laws with a simple majority (51 votes). In theory, a simple majority is enough to pass a bill, but if a Senator introduces a filibuster, an extra 60 votes are needed to override it and stop debate so a vote on legislation can be held.

Still, just having a Senate majority is crucial, particularly if there is a tie-breaking vote. (The vice-president is president of the Senate and only has a vote if the vote is tied).

Here are four key ways in which who wins the Senate matters.

1. Legislative agenda

Both the Harris and Trump campaigns have laid out sweeping proposals, especially for the economy, much of which will require Senate backing. While a filibuster-proof 60 votes is usually needed to pass laws, a special process called “budget reconciliation” can (with the consent of the official in charge of the rules, the Senate parliamentarian) be used to approve some budgets – relating to specific tax, spending and debt bills – with a minimum of a tie-breaking majority.

Harris’s plan focuses on building what she calls an “opportunity economy,” which includes US$25,000 (£19,200) in down-payment assistance for first-time homebuyers, US$6,000 tax credits for families with newborns, and federal bans against excessive prices for food and other groceries. Harris has also pledged to raise the corporate tax rate from 21% to 28%, and floated taxing unrealised gains – such as the appreciation in equities, real estate and other assets – for the very rich, a 25% minimum tax on total income exceeding US$100 million.

Trump’s economic blueprint includes making his 2017 tax cuts permanent. He’s called for the elimination of taxes on tips, overtime, and social security benefits. Additionally, Trump has vowed to slash the corporate tax rate from 21% to 15%. Perhaps Trump’s most consequential economic proposal – imposing 10-20% tariffs on all imports into the US and 60% tariffs on goods from China – could be done unilaterally without Congress.

2. Supreme Court

Some of the biggest battles over the next four years are likely to be fought in, and over, the federal judicial system. The Senate must consent to Supreme Court appointments. During his first term, Trump pushed through three court appointments – Neil Gorsuch, Brett Kavanaugh and Amy Coney Barrett – which helped solidify a six-three conservative supermajority on the bench. Biden named one justice, Ketanji Brown Jackson.

While no justice has signalled an intent to step down soon, either Trump or Harris could have the opportunity – planned or unplanned – to install one or more new justices. The two oldest-serving members of the court are conservatives Clarence Thomas, 76, and Samuel Alito, 74. For Republicans, the next presidential term could offer an opportunity to cement a right-leaning bench for decades to come.

If Trump wins and the Senate goes Republican, there will be pressure from conservative corners for the older right-leaning justices to retire and to replace them with young blood. By contrast, if Harris wins and the Democrats control the tiebreak, they could begin to redirect a court that’s been drifting rightward for years.

3. Future of the filibuster

Left-wing Congress members have advocated for ending the filibuster throughout President Joe Biden’s term. This “nuclear” option would mean doing away with a Senate rule, which was used in the first Congress in 1789. Ending the filibuster would signal an all-out partisan war that would have wide-ranging ramifications on Capitol Hill not only for the next presidency, but further into the future.

The filibuster has already been diluted in recent years by both Democrats and Republicans. In 2013, Democrats removed the 60-vote threshold to confirm many executive branch nominations, a move they said was necessary due to Republican blockading. In 2017, Republicans responded by killing the filibuster over Supreme Court appointments.

If elected, Harris has indicated that she would support ending the filibuster to reinstate reproductive rights that were eliminated after the overturning of Roe v Wade. However, she has talked little about the issue since becoming the Democratic nominee for president. It’s also unclear that more centrist Democrats would support the move.

4. Foreign policy

While there’s bipartisan support in Washington for both aiding Israel’s military and taking a “tough on China” approach, the incoming Senate will be essential in determining if the US approves additional funds to Ukraine.

With the retirement of Republican minority leader Mitch McConnell, a vocal advocate for supporting the war, it’s unclear if such a measure would even come up for a vote under Republican leadership. But a Harris administration or a Democrat-led House or Senate, or both, would continue to lobby for US funding.

One important, but less-discussed, issue that may also arise before the Senate is the ratification of a defence pact between the US and Saudi Arabia. Both the Trump and Biden administrations have envisioned a Saudi-Israel deal normalising relations between the two countries, with a US security pact for Saudi Arabia to back the agreement.

Any future treaty would require a two-thirds Senate majority, a high bar to clear. Twenty Democratic senators raised concerns to Biden about the potential deal in 2023, while Republican senators voted to block Trump’s proposed armed sales to the Saudis in 2019.

Both at home and abroad, it’s not just who wins the White House that will determine the political trajectory of the United States. Races in the Senate could have far-reaching implications under either a President Harris or President Trump.

The authors do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.