As many concerned U.S. election watchers continually refresh their 538 forecast in the final days before polls close, their attention tends to focus on the presidential race between Kamala Harris and Donald Trump, but the election website's predictions for the battle for the House and Senate are falling presents an even more worrying picture for the 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 smaller 52% chance of retaining their majority in the House.

These numbers reflect a reality that frightens left-leaning Americans: Republicans have a good chance of winning not only the White House but also full control of Congress.

Even without majorities in both houses of Congress, Trump's victory in the presidential race would give him significant control over U.S. foreign policy and the makeup of the federal government, both of which he wants to reform.

But a trifecta of Republicans in Washington would give Trump much broader power to implement his legislative agenda. As the Guardian has outlined as part of the Stakes Project, Trump's plans include extending tax cuts, rolling back landmark legislation signed by Joe Biden and promoting a right-wing cultural agenda.

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

Perhaps the scariest possibility for Democrats is that Republicans would use their power triangle in Washington to enact a nationwide abortion ban. Trump has said he would veto such a policy, but his repeated shifts on the issue have cast doubt on that claim. Research has shown that existing abortion bans force doctors to provide substandard medical care and are 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 devote enormous resources to support Trump's plan to deport millions of illegal migrants, which has become a central plank of his reelection campaign. While U.S. courts have confirmed that presidents have wide latitude in setting immigration policy, Trump will rely on Congress to allocate significant funding 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 November 5, 2024 will be Liberation Day in America.”

In addition to advancing Trump's agenda, Republicans would almost certainly seek to unravel key parts of Biden's legacy, including the Inflation Reduction Act. The IRA was the country's most significant response to the climate crisis to date and spurred significant energy investments in many counties, prompting some Republicans to suggest that Congress should keep some provisions of the law in place and repeal others.

This dilemma reflects a potential problem for Republicans if they win full control of Congress: What will they do with the Affordable Care Act (ACA)? The last time Republicans held a government triangle during Trump's first two years in office, they tried to repeal and replace the ACA but failed. According to a video published by NBC News, Republican House Speaker Mike Johnson recently suggested that there would be “no Obamacare” if his party won a big victory on Tuesday.

But he appeared to qualify that statement by telling his supporters: “The ACA is so entrenched that we need massive reforms to make this work, and we have lots of ideas about how to make that happen.”

In recent years, both parties have had to govern with narrow majorities in Congress, and election experts generally expect the battle for the House and Senate to be particularly tough this year. During Biden's first two years in office, his legislative proposals were repeatedly blocked in the Senate, even though Democrats had a majority because of concerns from two centrist members of their caucus, Joe Manchin of West Virginia and Kyrsten Sinema of Arizona.

In 2017, when Republicans had a 52-48 majority in the Senate, 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 — still serve in the Senate today and may oppose various components of Trump's agenda, particularly a possible abortion ban.

Despite the potential challenge of narrow majorities, Trump has made it clear at every turn that he will maximize his power as president if he wins on Tuesday.

“With your vote in November, we will fire Kamala and save America,” Trump said at his recent rally in State College, Pennsylvania. “We will never give in and we will never give up.”

Voters will have the final say on Tuesday on how much power Trump and his party will receive in January.

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