President-elect Joe Biden's agenda in Washington may only go as far as Mitch McConnell lets him.

McConnell, the Senate Republican leader who was reelected in Kentucky to a seventh term last week, is poised to hold the key to Biden's legislative agenda no matter what happens in two likely runoff elections in Georgia in January that will determine the party that controls the chamber.

And that means Biden will immediately have to confront the reality that could force bipartisan deals -- which could frustrate the left -- or face a wall of GOP opposition led by a shrewd Senate leader who has long preached party unity in the face of his Democratic adversaries. Either way, McConnell is bound to substantially limit the scope of the Democratic agenda, whether it's on immigration or climate change, and could affect how Biden goes about making crucial appointments to federal courts and his Cabinet.

But one dynamic could be in play to alter the typical gridlock that has dominated Washington in recent years: Biden and McConnell have a history of cutting deals together and have developed a bond that they say is built on trust after serving in the chamber together for decades.

Democrats are sceptical.

"No matter what happens in Georgia, it's going to be really hard to get anything passed in the Senate," Sen. Chris Murphy, a Connecticut Democrat, told CNN. "What I really worry about is an instant constitutional crisis where Mitch McConnell refuses to confirm any of Joe Biden's nominees unless they have received a personal stamp of approval from Mitch McConnell."

Illinois Sen. Dick Durbin, the No. 2 Democrat, put it bluntly: "Which Mitch McConnell will we get? ... If he's willing to sit down with Biden, if he feels he must, it may lead to something bipartisan and positive. If he takes the other approach, we are preparing for the next presidential election, we're going to have as an empty agenda in the Senate as we've had for the last few years."

McConnell confidantes say it's not so simplistic. They contend that it will be Biden's decision on how to govern: Work with Republican senators to find bipartisan consensus -- or cater to the left-wing and House Democrats, something that would lead to the likelihood of a stymied agenda.

"Make no mistake: If Mitch McConnell remains majority leader of the United States Senate, he will be able to check the worst impulses of this national Democratic Party," said Sen. Todd Young, an Indiana Republican who chairs the Senate GOP's campaign arm.

Navigating the Senate, a body where Biden served for 36 years, will be the biggest challenge for the incoming President. Biden will be pressured by Democrats in the House to push forward an ambitious agenda, even if it stands no chance in the Senate. And if he cuts deals with McConnell, it's bound to prompt a backlash from his liberal supporters.

Rep. Pramila Jayapal, who co-chairs the House's progressive caucus, told CNN that Biden should not back down from a "bold" agenda no matter what McConnell might prefer. In contrast, the other chair of the caucus, Wisconsin Rep. Mark Pocan, said: "I think people want us to do something -- period. And I don't know how you break the problem of the Republican Senate."

"If Mitch McConnell's going to go back into his shell, and we're not going to have legislation move, well then that's a different calculation," Pocan added.

The issues will be difficult for the two parties to resolve -- particularly on health care if the Supreme Court strikes down the Affordable Care Act. Moreover, lawmakers are confronting an economic stimulus package to deal with the impact of the coronavirus, a top priority in a lame-duck session of Congress that begins Monday. Other issues like infrastructure spending and dealing with prescription drug prices have generated bipartisan interest, but the details have long confounded the two parties.

Even Republicans who were critical of President Donald Trump issued a warning to Biden on Sunday.

"It's pretty clear they don't want the Green New Deal. It's pretty clear they don't want Medicare for All. They don't want higher taxes. They don't want to get rid of oil, and gas and coal," Utah Sen. Mitt Romney, a frequent Trump critic, said of US voters during an interview Sunday on CNN's "State of the Union." "Any argument to the contrary will be met with a lot of resistance from the American people and from members of Congress."

Depending on what happens with the two Georgia Senate races -- one of which has already advanced to a January 5 runoff and the other that is likely to -- McConnell will either be in charge of the Senate's agenda as majority leader, holding either a 52-48 or a 51-49 advantage. Or, he'll be in the minority with a Senate evenly split at 50-50, where Vice President-elect Kamala Harris would break tie votes.

Even if he's in the minority, Republicans would have the power to stop legislation in its tracks by using the filibuster, something that requires 60 votes to overcome. And Democrats fully recognize that a 50-50 Senate is unlikely to change filibuster rules because several Democratic senators are opposed to making changes to the potent stall tactic. (They would need 50 senators to agree to change the filibuster rules, with the vice president breaking the tie.)

"We are divided as divided can be, and I've always said that I'm not inclined to do anything that would divide us more," Sen. Joe Manchin, a West Virginia Democrat who opposes changes to the filibuster rules, said in an interview.

Murphy said if Democrats had 53 or 54 seats, there would be a "decent chance" of gutting the filibuster. But he added: "If the majority is 50, that's a lot more difficult to eliminate the filibuster."

Democrats are anticipating tension within their party.

"It's always hard," Durbin said of getting legislation through the Senate. "There are always going to be two or three people who say, 'Woah, you're moving too fast. You're moving too far to the left. You're moving on issues that don't help me back home.'"

Durbin said: "The natural inclination is to play to the folks in the most vulnerable position. Of course you love them, and you want to help them. But if you take that as your yardstick, you're going to have a very limited agenda."

If McConnell remains majority leader, he could essentially have veto power over legislation in addition to nominations to Biden's Cabinet and federal courts, which require a majority of the chamber to overcome a filibuster. And McConnell will have a major say in how legislation is shaped -- unless 10 Republicans break ranks, something challenging in a polarized body unless the GOP leader gives his blessing.

All that is a recipe for deal-making between McConnell and Biden -- who reached a series of critical spending and tax agreements during Barack Obama's presidency, including preventing the so-called fiscal cliff that could have imperiled the economy after the 2012 elections. But such an approach could frustrate Democrats since any bipartisan deal may fall short of the goals of the left.

McConnell and Biden were forced to work together because there was bad blood between McConnell and Obama, whom the GOP leader vowed to make a one-term president after he was elected -- a public pronouncement many Democrats never forgave.

In a 2016 video for Business Insider, McConnell was unvarnished about his distaste for negotiating with Obama who McConnell said was "grating and irritating" because he "thought he was the smartest guy in the room and needed to share that with you on a frequent basis."

"The guy to negotiate with in the administration was the Vice President, not the President," McConnell said. "With Biden, we didn't waste a lot of time talking about things we knew we would never agree on. I didn't lecture him, he didn't lecture me. We got down to the areas where there was a possible agreement and were able to get to an outcome. A very different experience from being in a negotiating setting with the President."

Four years ago, at the end of Biden's term as vice president, McConnell joined senators of both parties in a moving tribute to Biden on the Senate floor.

"You've been a real friend. You've been a trusted partner, and it's been an honour to serve with you. We are all going to miss you," McConnell told Biden with emotion.

Little did anyone realise their association would be renewed four years later with Biden returning to the White House as President and in need of a Republican partner in the Senate to help him advance his agenda.

text by Manu Raju and Ted Barrett, CNN