In the 100-seat Senate, only 35 seats are up for election this year, and there are only a handful of closely contested races. A net change of one seat toward the Republicans would give them control.
Although millions of votes are yet to be counted in the midterm elections, but early estimates suggest the Republican Party is on course to win back control of the US House of Representatives for the first time in four years.
The upper chamber of Congress, the Senate, remains too close to call.
cAfter two years of unified Democratic control in Washington, the power dynamic in the nation's capital is poised to shift. Here are four very real implications for American politics for the next two years, BBC reports.
In their two years in office, Joe Biden and the Democrats were able to enact a fairly substantive agenda, which included massive spending on the environment, healthcare and other social programmes.
That would all end with a Republican victory.
There's the chance of some co-operation - for instance, Republicans and Democrats did join together to pass gun control and technology investment this year and infrastructure spending last year. However, big-ticket liberal priorities on abortion, education and voting rights will be dead in the water.
Republicans have their own agenda, focused on border security, law enforcement spending, budget cuts and fossil fuel extraction. But even if the Republicans take both chambers of Congress, Democrats will be able block passage in the Senate or in the White House using Mr Biden's veto power.
For the next two years, legislative gridlock will be the name of game.
For two years, Democrats have been calling the shots - that's meant an expansive investigation into the 6 January, 2021, attack on the US Capitol, and hearings on subjects including abortion, healthcare and voting rights.
If Republicans take control of congressional committees, the priorities will rapidly shift.
House conservatives have already promised a hearing into Joe Biden's son Hunter's business ties to China. They also want to look into the Biden administration's immigration policies, the US withdrawal from Afghanistan and the origins of the coronavirus pandemic in China.
The Senate Judiciary Committee handles review of presidential nominations to the federal courts. For the past two years, Democrats have set a modern record for the number of new judges seated to lifetime appointments.
If Republicans also take control of the US Senate, expect the process for confirming Mr Biden's nominees to come to a standstill. And if a Supreme Court vacancy opens up, there's a good chance it could remain unfilled until the next presidential election.
With Democratic control of Congress, the US had a two-year reprieve from the game of chicken that have led to government shutdowns and near default on the national debt. That's about to end.
Some Republicans, including Congressman Kevin McCarthy who is poised to become House speaker if Republicans take the chamber, are already threatening to force Democrats to agree to sweeping budget cuts.
The US has never defaulted on its debt. However partial government shutdowns due to the inability of Congress to approve annual spending legislation have become more common. It happened twice during the Trump administration and once under President Barack Obama.
If Republicans and Democrats can't agree on a basic framework for government spending, another government shutdown by the end of next year seems highly probable.