There is a new group of political power brokers in Washington.

They will not be found in the White House or government departments, nor in the ranks of Congressional leadership, but rather in the US Senate, where a handful of centrist lawmakers are ready to wield outsized influence.

Their newfound power comes as the Democrats prepare to take control of the upper chamber of Congress by the narrowest of margins after winning two run-off elections in Georgia earlier this month. The 100-member Senate will be split 50-50, with Kamala Harris, the new US vice-president, able to cast a tiebreaking vote.

Given the tight margins, a group of self-styled moderates that do not always vote along party lines will prove crucial to Joe Biden’s chances of passing legislation, most pressingly his $1.9tn Covid-19 relief package.

The group includes Democrats Joe Manchin of West Virginia and Arizona’s Kyrsten Sinema, as well as Republicans Lisa Murkowski of Alaska and Maine’s Susan Collins.

“They become the dealmakers, or the deal-breakers, depending on what the bill is,” said Republican strategist Doug Heye. “Clearly, they are in a position of influence that is much greater than just a week ago.”

Democrats still control the agenda, with Chuck Schumer, the senator from New York, becoming Senate majority leader, and Mitch McConnell, the upper chamber’s top Republican, demoted to minority leader. At the same time, Nancy Pelosi, the Democratic speaker of the House of Representatives, will maintain control of the lower chamber.

The Senate has been split evenly just once before, in 2001, when Dick Cheney, George W Bush’s vice-president, was able to cast the tiebreaking vote for Republicans.

But that changed in June that year when Jim Jeffords, a senator from Vermont, announced he was leaving the Republican party and would caucus with the Democrats, leaving the chamber 51-49 in Democrats’ hands. The switch made Tom Daschle, the Democratic senator from South Dakota, the Senate majority leader.

Mr Daschle, who left the Senate in 2005, said in an interview with the Financial Times this week that he thought it was a “good thing” that Mr Biden’s agenda would depend on the backing of people such as Mr Manchin, a self-described “conservative Democrat” from West Virginia, a state that Donald Trump won by an almost 40-point margin in November.

Mr Manchin voted more times in favour of the Trump agenda than any other Democratic lawmaker in recent years. He has sided with Republicans on abortion and voted in favour of Brett Kavanaugh’s nomination to the Supreme Court. But he voted alongside Democrats in opposing Mr Trump’s tax cuts and supporting Barack Obama’s Affordable Care Act.

“They will be the ones who can ensure that there is at least an effort to reach compromise,” Mr Daschle said of Mr Manchin and other moderate Democrats such as Ms Sinema and Mark Kelly, another Arizona senator.

“I have always said that compromise is the oxygen of democracy, and we need a lot more oxygen in the Senate right now. I think they can be the ones to provide it,” Mr Daschle said.

Others are more sceptical, especially given how sharp the partisan divide in Washington has become.

Mr Biden takes office under the long shadow of an impeachment trial for Mr Trump, and it remains unclear whether more than a dozen Republican senators will break with their party and vote to convict the outgoing president for inciting the violent January 6 insurrection on Capitol Hill.

“It is a very different environment right now,” Mr Heye said. “Given the polarisation in the country, which manifests itself in both the House and the Senate, it is going to be hard to get things done.”

So far, a handful of Republicans, including former presidential candidate Mitt Romney, who was the only GOP lawmaker to vote to convict Mr Trump at his first impeachment trial, have indicated they are open to conviction.

While Mr Biden may use a process called budget reconciliation to push through the bulk of his economic relief plans through the Senate with a simple majority, most of his other legislative proposals will need the backing of at least 60 senators under arcane “filibuster” rules.

“Nothing happens in the Senate without . . . collaboration unless you have that very rare and fleeting 60 votes,” said John Lawrence, a veteran congressional staffer who was chief of staff to Ms Pelosi from 2005 to 2013. “Ultimately, the Senate is the keyhole that any proposal has to be able to slot through.”

Progressives have called for the scrapping of the filibuster to make it easier for Democrats to push through everything from aggressive policies to tackle climate change to statehood for the District of Columbia. But Mr Manchin has already said he is against filibuster reform, making it extremely unlikely.

That means Mr Schumer will not only need to hold on to votes from Mr Manchin and other moderate Democrats, but also win over as many as 10 Republicans in order to pass major pieces of legislation — no small feat in the current political climate.

Some Democrats hope they will be able to count on support for some legislation from Ms Murkowski, who said last week that the House acted “appropriately” by impeaching Mr Trump. In a recent interview with the Anchorage Daily News, she said “if the Republican party has become nothing more than the party of Trump, I sincerely question whether this is the party for me”.

But Washington stalwarts caution Mr Biden will need the backing of more than one or two Republicans to make his plans “filibuster-proof”.

“Getting three or four [Republicans] is reasonably likely,” said Matt Bennett, co-founder of the centrist Democratic think-tank Third Way. “Getting 10 of them is very, very hard.”