Democrats have run the U.S. House of Representatives since the beginning of 2019. At noon on Jan. 20, Joe Biden, a Democrat, becomes president. That means that -- once Democrats secure their ever-so-narrow majority in the Senate -- they will control both the executive and legislative branches for the first time since 2011. Exactly when that happens involves multiple moving parts that may influence, among other things, when the House-approved impeachment article against outgoing President Donald Trump is sent to the Senate for a trial.

1. Who controls the Senate now?

Since Jan. 3, when the 117th Congress was sworn in, the Senate’s composition has been 51 Republicans, 46 Democrats and two independents who vote with Democrats. That’s why the top Senate Republican, Mitch McConnell, has remained majority leader. Since Georgia had two runoff elections on Jan. 5, one of that state’s Senate seats is temporarily vacant, and the other will be held for a few more days by Kelly Loeffler, a Republican. The two Democrats who triumphed in the runoffs, Jon Ossoff and Raphael Warnock, will be seated once that election result is certified.

2. When will Ossoff and Warnock be seated?

Georgia election officials expect to certify the election results by end-of-day on Jan. 20, the day of Biden’s inauguration. Ossoff and Warnock then must bring their certification papers to Washington to be sworn in. Such swearings-in often occur the day after state certification. The addition of Ossoff and Warnock would bring the Senate’s balance of power to an even split, 50-50.

3. Who controls a 50-50 Senate?

The party of the vice president, who acts as president of the Senate and breaks ties when they occur. That will be the Democratic Party as of midday on Jan. 20, when Kamala Harris is inaugurated as vice president. But therein lies a twist, since Harris must resign her seat as a U.S. senator from California -- reducing, for a short time, the number of Democratic senators.

4. How quickly does Harris’s seat get filled?

This depends on when she resigns. Previous senators elected to the vice presidency have resigned from the chamber before Inauguration Day. If Harris does so, her replacement could be sworn in before Jan. 20. California Governor Gavin Newsom picked his Secretary of State Alex Padilla for that seat. Alternatively, Padilla could be sworn in shortly after the inauguration. The current plan, according to a person familiar with the discussions, is for Padilla to be sworn in on the Senate floor minutes after the inauguration.

5. So, when do Democrats take charge?

Once the three new Democratic senators and Harris have been sworn in -- possibly on Jan. 20, but possibly a day or two later, depending on the timing of Georgia’s certification -- Democrats will officially have the Senate majority, and with it the power to set the chamber’s agenda, including confirmation of Biden’s first nominees. That prerogative will shift from McConnell to Chuck Schumer, the Democratic leader in the Senate. “Schumer automatically becomes majority leader at the time the Senate is 50-50 and Harris is vice president,” said Steven Smith, a political science professor at Washington University in St. Louis who specializes in congressional politics.

6. How will the Senate work with a 50-50 split?

Even with the slimmest possible margin, Schumer will enjoy most but not all of the powers of any Senate majority leader. When the Senate was evenly divided at the start of 2001, its Republican and Democratic leaders -- Trent Lott of Mississippi and Tom Daschle of South Dakota -- negotiated a power-sharing agreement that provided for equal membership on committees, equal budgets for committee Republicans and Democrats, and the right of either party leader to discharge bills or nominations from committees that deadlocked on whether to advance them. McConnell and Schumer are using that pact as a framework for their own plan, which remains a work in progress. The 2001 power-sharing agreement “serves as a template, but the partisan distrust runs much deeper now,” Smith said. “This will be tricky and turn mostly on the leaders’ commitment to finding an agreement while under pressure from party colleagues to gain an edge over the other side.”

7. What does this mean for Trump’s impeachment?

The looming shift in Senate control is among the factors that may influence what happens next with the article of impeachment approved by the House on Jan. 13. It charges Trump with incitement of insurrection in connection with the Jan. 6 riot at the Capitol. House Speaker Nancy Pelosi hasn’t said publicly when she’ll send the article to the Senate for a trial. Once that happens, all other business in the chamber generally grinds to a halt unless there is a deal to conduct other work. That could mean no confirmation votes for Biden’s nominees or votes on early legislative initiatives. Biden has said he’s asked the Senate parliamentarian and Senate leaders if it would be possible for the Senate to consider his nominees and perhaps legislation during an impeachment trial, with half days devoted to the trial proceedings and the rest to other Senate work. The matter hasn’t been resolved, and McConnell’s aides have not said whether he would endorse or oppose that approach despite repeated inquiries. Assuming all 50 Senate Democrats vote to convict, they will need the support of 17 Republicans to reach the required two-thirds supermajority to make Trump the first president ever convicted in an impeachment process.

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