If incumbents in North Carolina and Alaska hold their current leads, Republicans will have 50 Senate seats in the next Congress. But they need 51 for a majority to organize the body because Vice President-elect Kamala Harris will preside over the Senate. She would cast the deciding vote in a 50-50 Senate.

What difference would a single vote make? Republicans would lose their committee chairmanships and thus the power to serve as a check on the Biden Administration. Joe Biden deserves the Cabinet he wants in most cases, but a GOP Senate could deter appointments like Elizabeth Warren at Treasury. Oversight Chairman Ron Johnson’s probe of FBI and other abuses would cease.

Or consider the Democrats poised to run key committees if they organize the Senate. Bernie Sanders would run Budget, which means a squeeze on the Pentagon. Sherrod Brown of Ohio would run Banking, and Ms. Warren would run the financial institutions subcommittee. Have fun, bankers.

Oregon’s Ron Wyden would run Finance. He supports the $4 trillion Biden tax increase plus he wants to tax even unrealized capital gains as ordinary income. That means taxing the appreciation in the value of assets even if they aren’t sold during the year. He isn’t kidding.

Some business folks think nothing like this could happen in a 50-50 Senate. West Virginia Democrat Joe Manchin would supposedly save the day, or Joe Biden would intervene. Don’t count on it. Mr. Manchin is a reliable Democratic vote on every big issue when it really matters, and Mr. Biden will be under pressure from progressives (and Ms. Harris) to achieve their goals.

Even without breaking the 60-vote legislative filibuster rule, Democrats could pass a budget under reconciliation rules with 50 votes plus the Vice President to make 51. This can include spending and tax measures. Most of the Biden plan would pass because Democrats want the revenue to spend.

A 50-seat Democratic majority led by Mr. Schumer might also be willing to break the filibuster. The New Yorker fears a primary challenge in 2022 from Rep. Alexandria Ocasio-Cortez, who has a national following and could raise tens of millions of dollars if Mr. Schumer disappoints progressives. A Senate platform would vault AOC into the mix for a presidential run in 2024.

All of this should be front-and-center for voters in the Georgia runoffs. The suburban Atlanta Republicans and independents who voted for Mr. Biden to oust Donald Trump are likely to support a GOP Senate as a check on Mr. Biden’s left wing if they know the stakes.

Mr. Trump and his allies should also keep the Georgia runoffs in mind as they fight legal battles over the presidential race. A bitter-end attempt to urge state legislators to override state ballot counts in awarding Electoral College votes could backfire with voters who otherwise would vote for divided government. Mr. Trump’s future influence will diminish if he costs Republicans the Senate majority as he leaves the White House. The Georgia stakes couldn’t be higher.