For President-elect Joe Biden, Santa Claus came a few weeks late, but he certainly delivered. Georgia's Senate races give Democrats control of both houses of Congress — a spectacular gift.

I worked in the Barack Obama administration from 2009 to 2012, and I was able to see, close up, the staggering difference it makes when the Senate and the House are controlled by the same party as the president.

That was the case in 2009 and 2010, when Congress enacted not only the American Recovery and Reinvestment Act (the economic stimulus made necessary by the 2008 financial crisis), the Affordable Care Act, and the Dodd-Frank banking reforms — but also the Family Smoking Prevention and Control Act, the Credit Card Accountability Responsibility and Disclosure Act, and the Lily Ledbetter Fair Pay Act, which strengthened the available tools to combat employment discrimination in court. This was one of the most consequential periods of lawmaking in the nation's entire history.

Everything changed in 2011, when Republicans won a majority in the House. That meant that in 2011 and 2012, Congress wasn't going to do much, especially if Obama favored it.

For many issues, executive actions became the only game in town.

Actually, it was far worse than that. Republican-controlled committees spent a lot of their time hurling accusations and launching investigations, sometimes alleging violations of the law. Instead of working with Congress to address problems, the White House was often on the defensive, responding to the latest attack.

In the weeks before Tuesday's runoff elections in Georgia, it appeared likely that Republicans would retain control of the Senate, which would mean a repeat of 2011 and 2012, and possibly worse: American politics is even more polarized now than it was when I worked in Washington, and the Republican Party even less cooperative. For any president, losing the Senate is worse than losing the House, because the Senate has the power to stop presidential appointments.

With the apparent results in Georgia, the ground has shifted. Here are the major consequences:

• Judicial appointments will be a lot easier. Sen. Mitch McConnell has been a master of blocking choices by Democratic presidents.

• Biden will be able to get his own team in place much more rapidly. Members of the Cabinet are Senate-confirmed, of course, but they're only the tip of the iceberg. Biden will be choosing deputy secretaries, assistant secretaries, general counsels and many others. Senate Republicans will do whatever they can to slow things down, but Biden will be in a strong position to create a well-functioning executive branch.

• The White House counsel, and the White House generally, won't have to fend off a host of investigations. Under Republican leadership, there was a real chance that Washington would be preoccupied with all sorts of Senate scandal-mongering: inquiries into the legitimacy of Biden's victory, into the finances of his son Hunter Biden, and into conspiracy theories of multiple kinds. Almost all of that is suddenly off the table.

• Oh, and a significant stimulus helping small businesses, large businesses, the elderly, the sick and others is suddenly on the table.

You would be right to say that the Republicans can resort to the filibuster (more on that later), which would require 60 votes, which Democrats will not often have. But through a process called reconciliation, the Senate can act by a bare majority on many things.

The scope of the reconciliation process is disputed, and some of the issues are arcane. It is designed for issues that involve the budget, which means taxing and spending. But a lot of reforms involve those things, and it was used to help turn the Affordable Care Act into law. Biden and the Democrats can use reconciliation to enact a massive economic package, and to do it quickly.

A new stimulus package could simultaneously address some of Biden's core concerns: climate change, health care, economic equality and racial justice. Alternatively, Biden could consider new legislation, also with the help of the reconciliation process, on one or more of those subjects.

Tax reform would also be a prime candidate. Biden's highly progressive reform plan may or may not get 50 Democratic votes, but it could easily be the starting point for legislation.

Before the Georgia vote, it would have been reasonable to say that Biden's options for addressing climate change were limited to existing regulatory tools — allowing restrictions on emissions from (for example) power plants and motor vehicles. Those are powerful tools, but his preferred program is much more ambitious. It includes rebuilding infrastructure, upgrading buildings, spurring new construction and promoting clean-energy technologies. All that could happen through reconciliation, now that both houses of Congress are in Democratic hands.

Still, it's not going to be smooth sailing.

When I joined the Obama administration in January 2009, we had a strong Democratic majority (56 Democrats along with two independents who caucused with them), and for the first two years, Republicans had between 39 and 42 seats. Even so, Obama struggled to get legislation enacted, partly because of Republican intransigence and guile and partly because of the diversity of views among Democrats, which made it hard to reach consensus. Biden will confront those problems as well.

The reconciliation process could not be used for many of Biden's priorities, such as immigration reform, meaning that Republican filibusters could force Democrats to find 60 votes for measures that many of them ardently favor, something that will sometimes be impossible to do.

Should the Democrats eliminate the filibuster? That's a tough one, and they might not have the votes, anyway.

Even so, the world has changed. All of a sudden, Biden's largest ambitions have started to look realistic.

Cass R. Sunstein is the author of "Too Much Information" and a co-author of "Nudge: Improving Decisions About Health, Wealth and Happiness."