• The undecided Senate races in Georgia are "the most important Senate elections in modern times," GOP pollster and strategist Frank Luntz told CNBC.
• Both Senate seats are held by Republicans. One of the races is headed to a Jan. 5 runoff, while other is too close to call, according to NBC News.
• "If the Democrats can win both of these seats back, then they will be in control of the Senate" as well as the House and the White House, Luntz noted.

Republican pollster and strategist Frank Luntz told CNBC on Wednesday that the undecided U.S. Senate elections in Georgia are critical in shaping the trajectory of President-elect Joe Biden's time in the White House, calling them "the most important Senate elections in modern times."

The race between GOP Sen. Kelly Loeffler and Democrat Raphael Warnock is headed to a runoff on Jan. 5, after neither candidate failed to secure 50% of the vote in last week's general election. The state's other Senate contest, between Republican Sen. David Perdue and Democratic challenger Jon Ossoff, also appears to be headed to a runoff — although NBC News has not made an official call.

The balance of power in the Senate will be at stake if both end up heading to a January runoff — and if both were to flip Democratic, then the Senate would be split down the middle.

Republicans currently hold 50 seats, compared with 48 in Democratic hands, including two independent senators, Bernie Sanders of Vermont and Angus King of Maine, who caucus with the Democrats. That means a pair of Democratic victories in Georgia would make the 100-seat Senate 50-50. Vice President-elect Kamala Harris would hold the tie-breaking vote.

"If the Democrats can win both of these seats back, then they will be in control of the Senate and the president will have an agenda that won't be able to be challenged" on Capitol Hill, Luntz said on "Squawk Box." "If the Republicans win even one of these seats, then they [retain] control the majority and they will determine exactly what of the president's agenda will go forward."

Democrats will keep control of the House, according to NBC News, which puts the count at 225 Democrats and 210 Republicans. That would be a net-loss of seven seats for the Democrats but still over the 218-seat threshold needed to keep their majority.

Luntz said the "consequences" of Democrats winning both Senate seats in Georgia make it more likely that ambitious proposals on climate change and rolling back portions of President Donald Trump's signature tax reform law could be approved. Democrats also could push for changes to Senate rules, such as eliminating the filibuster, which establishes a 60-vote threshold to bring legislation to the floor, according to Luntz.

However, Democratic Sen. Joe Manchin of West Virginia said this week that he would oppose proposals to end the filibuster or expand the number of justices on the Supreme Court, a pledge that, if fulfilled, would dash progressive hopes for more structural reforms.

Luntz noted Manchin's recent remarks, saying he takes the West Virginia Democrat at his word, calling him a "reasonable senator." Nonetheless, Luntz noted that Democratic control of the Senate would still mean that committee chairs would be considerably more liberal than Republican counterparts. "The stakes have never been so high and everybody in Washington is going to be impacted by what happens on Jan. 5," he said.

Even so, Luntz said he believes it is challenging to forecast what will happen in the Georgia Senate races — especially given the prospect that Biden would ultimately win the presidential contest there. With 99% of the expected ballots counted, Biden leads Trump by just over 14,000 votes of nearly 5 million cast. No Democratic presidential candidate has won Georgia since Bill Clinton in 1992. Now, however, it is a "genuine swing state," Luntz said.

However, later Wednesday morning after Luntz's interview, Georgia's Republican secretary of state, Brad Raffensperger, announced a statewide "full, by-hand" recount in the presidential election. He said he aims to finish the recount in time for the Nov. 20 deadline to certify the state's election results.

Among the factors that make the Georgia Senate outcomes hard to predict, according to Luntz, is the massive spending in the races. He said it is also unclear what role Trump will decide to play in the election. "We don't know if he's going to take his marbles and go home, whether he's going to be active" in campaigning for the GOP candidates, Luntz said. Republican voters may also feel dejected if Biden ends up winning the state and not show up for the January vote, he said.

By contrast, Luntz said he expects Biden and allies to be active in courting support and turn out for Warnock and, potentially, Ossoff. "They will do everything they absolutely can. They're going to flood the state," he said.