As Republicans fight to hold onto their Senate control, Democrats are angling to pick up at least five seats this November, but a net gain of four would mean the upper chamber would be divided 50-50 between the the two parties next year.

An evenly split Senate has been rare in the U.S.’s 240-year history. It occurred for the first time in 1881, again in 1953, and the last time it happened was just 16 years ago, as a result of the 2000 election.

Experts tracking this cycle’s competitive Senate races say a 50-50 split is a real possibility again. Were that to happen, the party that wins the White House would determine which party becomes the majority in the Senate -- because the vice president casts the tie-breaking vote.

“When you look at the number of seats that are up, it would not surprise me if the Democrats fell a little bit short, but they pick up a few,” said Lara Brown, interim director of The George Washington University’s Graduate School of Political Management.

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There’s a “non-trivial chance” Democrats could wind up with 50 seats in the Senate, she said, and a Vice President Tim Kaine as the tiebreaker.

Right now, Republicans hold 54 seats and Democrats hold 46 seats in the Senate. Of the 34 seats up for grabs, 24 are held by Republicans. About six states that have toss-up Senate races and there are two in which Democratic challengers appear to be leading their Republican incumbents. Some of the vulnerable incumbents include Sens. Kelly Ayotte, R-New Hampshire, Richard Burr, R-North Carolina, Ron Johnson, R-Wisconsin, Mark Kirk, R-Illinois, Pat Toomey, R-Pennsylvania and Roy Blunt, R-Missouri.

Before Congress went on their pre-election recess last month, a reporter asked Senate Majority Leader Mitch McConnell, R-Kentucky, about the prospects of a 50-50 split in the Senate next year. In his view, Senate leaders would deal with that outcome by using the same kind of power-sharing agreement that was reached the last time this happened.

“We know we had that 16 years ago -- the Senate, after the 2000 election, was 50-50,” he told reporters at a press conference. “I think if we ended up 50-50, that we would simply replicate what we did in 2000.”

After the 2000 election, then-Democratic Leader Tom Daschle acknowledged that incoming Vice President Dick Cheney, with his ability to cast tie-breaking votes, would technically make Republicans the majority. But because of the even split on Capitol Hill, Daschle demanded that all Senate committees be equally divided between each party’s members as well as their staffs and resources. He also considered pushing for Democrats to co-chair committees.

A few weeks before the inauguration, Daschle reached a power-sharing agreement with Republican Leader Trent Lott, R-Mississippi, that required that Senate committees have an equal number of members from each party with staffing and funding levels divided down the middle.

lott and daschle AP / CBS

The deal’s language also made clear that if the Senate’s 50-50 ratio changed at any point over the next two years, leadership would alter the chamber’s procedures and reverse control. That actually wound up happening only a few months later when Republican Sen. Jim Jeffords of Vermont became an independent and chose to caucus with the Democrats.

During the Bush administration’s two terms, Cheney cast eight tie-breaking votes in the Senate, including on one of the Bush tax cuts. He helped pass the $350 billion package 51-50 in May 2003 that reduced income tax rates, cut rates on capital gains and dividends, expanded the child tax credit and gave $20 billion in aid to states.

An evenly split Senate next year, however, likely won’t necessarily mean so many tie-breaking votes. The Senate has remained closely divided over the last eight years with parties switching control only once. In that period, Vice President Joe Biden has not cast a single tie-breaking vote and experts say that could be the case for his successor even with a 50-50 Senate.

“Even if it is 50-50, it’s not going to matter all that much,” Brown said, explaining that both parties would still face the hurdle of overcoming a filibuster.

Neither party appears to have a chance of picking up enough seats to equal the 60-member threshold needed to overcome procedural votes and advance legislation. Therefore, Kaine -- or Mike Pence if Donald Trump is elected president -- likely wouldn’t have the opportunity to cast too many tie-breaking votes.

Plus, Democrats could again face the dynamic of dealing with a Republican-controlled House even though House Minority Leader Nancy Pelosi recently expressed optimism about Democrats’ chances for taking back the majority.

Because that wouldn’t be much of a change from the gridlock that has consumed Washington for the last decade, Clinton or Trump might not be able to accomplish big things legislatively.

“I think for policy legislation that is very partisan divided like immigration reform and gun control -- those are not going to come to a vote in the Senate,” Brown said. “Where I could see there being a policy where there might be a tie-breaking vote would be on something like trade, like a free-trade agreement.”

Experts said criminal justice reform and cybersecurity measures also could be issues on which both sides could find common ground and where the vice president’s tie-breaking vote might come in handy.

“My hypothesis is, Hillary, if she’s president, will go forward with things where she can find common ground -- cyber, maybe some tax reform. I don’t think she’ll go big,” at least initially, said James Thurber, professor of government at American University.

One legislative tool that Republicans took advantage of in the 2000s was budget reconciliation, which allows for passage on a majority vote and prevents the minority party from filibustering.

If the same party occupying the White House controls the Senate, they might try to use reconciliation again. Using that procedure could be difficult, however, depending on whether Republicans still control the House.

“In a period of unified control, the Senate majority party would not necessarily be slowed down by having a power-sharing agreement because they could count on the vice president for breaking a tie,” said Sarah Binder, senior fellow in governance studies at the Brookings Institution. “If they control the White House, you could imagine them using reconciliation to cut taxes or so forth. On nominations, unified party control will matter quite a bit because of that nuclear option a few years ago. Except for the Supreme Court, one can cut off debate on nomination with just 51 votes.”

While McConnell said they would likely repeat the power-sharing agreement of 2001 if they’re split 50-50, experts said it might not be that simple since Capitol Hill is much more polarized now than it was back then.

Of course, McConnell would also have to negotiate a deal with the likely next Democratic leader, Sen. Chuck Schumer of New York, and he might feel differently.

“There are various precedents the Senate can follow,” said Senate Historian Betty K. Koed. “We’ve had three pretty clear cases when we’ve had it evenly divided. Which precedent they will follow will be up to them or to set a new precedent will be up to them.”