As many anxious US election watchers constantly refresh the forecast from 538 in the final days before polls close, their attention tends to focus on the presidential race between Kamala Harris and Donald Trump, but the polling website’s forecasts of the battle for House and the Senate paint an even more worrying picture for Democrats.

As of Saturday evening, 538 gives Trump a 50% chance of winning the presidential race, while Republicans have a comfortable 90% chance of regaining control of the Senate and a narrower 52% chance of maintaining their House majority.

Those numbers reflect a reality that is chilling to left-leaning Americans: Republicans have a decent shot at winning not just the White House but full control of Congress.

Even without majorities in both chambers of Congress, Trump’s victory in the presidential race would give him significant control over US foreign policy and the makeup of the federal government, both of which he is seeking to overhaul.

But a Republican trifecta in Washington would give Trump much more sweeping power to implement his legislative agenda. As the Guardian has outlined through the Stakes project, Trump’s plans include extending tax cuts, rolling back landmark laws signed by Joe Biden and advancing a rightwing cultural agenda.

One of Republicans’ most oft-repeated campaign promises is that they will extend the tax cuts Trump signed into law in 2017, many of which are set to expire at the end of 2025. An analysis from the non-partisan Institute on Taxation and Economic Policy found that making the tax cuts permanent would cost $288.5bn in 2026 alone and disproportionately benefit the highest-income households. The highest-income 20% of Americans would receive nearly two-thirds of that tax benefit, compared with just 1% for the lowest-income 20% of Americans.

Perhaps the most haunting possibility for Democrats is that Republicans would use their governing trifecta in Washington to enact a nationwide abortion ban. Trump has said he would veto such a policy, but his repeated flip-flopping on the issue has raised questions about that claim. Research has shown that existing abortion bans have forced doctors to provide substandard medical care, and they have been blamed for the deaths of at least four women: Josseli Barnica, Nevaeh Crain, Candi Miller and Amber Thurman.

With majorities in both chambers, Republicans could also allocate vast resources to assist Trump’s plan to deport millions of undocumented migrants, which has become a central plank of his re-election platform. While US courts have affirmed that presidents have much leeway when it comes to setting immigration policies, Trump will need Congress to appropriate extensive funds to carry out such a massive deportation operation.

“The United States is now an occupied country,” Trump said at a recent rally in Atlanta. “But on November 5, 2024, that will be liberation day in America.”

In addition to advancing Trump’s agenda, Republicans would almost certainly be looking to unravel key portions of Biden’s legacy, including the Inflation Reduction Act. The IRA marked the country’s most significant response yet to the climate crisis and has spurred significant energy-related investments in many districts, prompting some Republicans to suggest that Congress should preserve some of the law’s provisions while repealing others.

That quandary reflects a potential problem for Republicans if they win full control of Congress: what will they do with the Affordable Care Act (ACA)? When Republicans last held a governing trifecta, during Trump’s first two years in office, they tried and failed to repeal and replace the ACA. The Republican House speaker, Mike Johnson, recently suggested that there would be “no Obamacare” if his party wins big on Tuesday, according to a video published by NBC News.

But he seemed to caveat that statement by telling supporters: “The ACA is so deeply ingrained, we need massive reform to make this work, and we got a lot of ideas on how to do that.”

In recent years, both parties have experienced the pains of governing with narrow congressional majorities, and election experts widely expect the battle for the House and Senate to be especially close this year. During Biden’s first two years in office, his legislative proposals were repeatedly blocked in the Senate despite Democrats holding a majority because of the concerns of two centrist members of their caucus, Joe Manchin of West Virginia and Kyrsten Sinema of Arizona.

We will guide you through the aftermath of the US election and the transition to a Trump presidency

after newsletter promotion

When Republicans held a 52-48 majority in the Senate in 2017, they still failed to repeal and replace the ACA because three members of their conference blocked the proposal. Two of those members – Susan Collins of Maine and Lisa Murkowski of Alaska – are still in the Senate today and may be resistant to various components of Trump’s agenda, particularly a potential abortion ban.

Despite the potential challenges of narrow majorities, Trump has made clear at every turn that he will use his presidential power to its maximum effect if he wins on Tuesday.

“With your vote this November, we’re going to fire Kamala and we are going to save America,” Trump said at his recent rally in State College, Pennsylvania. “We will never ever back down, and we will never surrender.”

The voters will have the final say on Tuesday to determine just how much power Trump and his party will have come January.

Read more of the Guardian’s 2024 US election coverage:

• US election 2024 live updates: latest polls, results and news

• When do polls close?

• How the electoral college works

• Where is abortion on the ballot?

• Senate and House races to watch

• Lessons from the key swing states

• Trump v Harris on key issues

• What’s at stake in this election

• What to know about the US election