WASHINGTON — When the 116th Congress convenes next week, Democrats will for the first time in eight years have the power to determine the House’s agenda, while Republicans in the Senate will have the power to protect President Donald Trump: he'll depend on them for his judicial nominees, his Cabinet picks — possibly, even his hold on the office itself.

Lawmakers are set to return to Washington for the new session on Jan. 3, when the House is scheduled to hold the formal floor vote expected to elect House Minority Leader Nancy Pelosi, D-Calif., speaker for the second time.

The balance of power in the House will shift from a 236-197 GOP majority to a 235-199 Democratic advantage, a swing of 40 seats. In the Senate, Republicans — who had struggled with a razor-thin 51-49 majority — will operate with a slightly more comfortable 53-47 margin.

"The time for endless delay and obstruction has come to a close," Senate Majority Leader Mitch McConnell said Monday in insisting that the Senate would vote this week on Brett Cavanaugh's nomination to the Supreme Court. Brendan Smialowski / AFP - Getty Images file

If Trump and Hill Democrats don't reach an agreement this week on border wall funding, the first big task of the new session will involve leftover business from the last: passing the remaining spending measures to end the partial government shutdown. And it doesn't get easier from there.

Here's some of the other action to watch for starting next week:

The Democratic agenda

House Democrats have said their agenda for the new year will include lowering the cost of prescription drugs, passing ethics reform, protecting Obamacare and crafting an infrastructure package.

The first bill they plan to unveil is H.R. 1, which seeks to update the Voting Rights Act in order to make the voting process easier, require presidential candidates to disclose their tax returns and encourage states to adopt public campaign finance systems, among other things.

The more-comfortable Republican advantage in the Senate and control of the White House means top Democratic priorities are unlikely to make it much beyond the House side of the Capitol — but leadership is determined to give members more to run on in 2020 than presidential oversight.

Trump’s tax returns

Still, Democrats do have a long Trump oversight to-do list, topped by one big task: obtaining the president’s tax returns.

Ways and Means is one of three congressional committees that has the authority to seek to obtain Trump’s tax returns, which he has refused to release since his presidential campaign because he claims that he is under audit by the IRS. The other two panels that have the ability to request a person’s tax returns under the tax code are the Senate Finance Committee and the Joint Committee on Taxation.

Pelosi recently reiterated that it will be up to the Ways and Means panel, set to be chaired by Rep. Richard Neal, D-Mass, to decide whether to request Trump’s returns.

Neal could submit a written request to the IRS to provide the information. If the Treasury Department were to deny it, House Democrats would have to decide whether to pursue the tax returns through a legal route. If they are obtained, the chairman would have to designate the panel’s members as “agents” to read the returns. They would then have to vote to make the documents public and report them to the full House.

Trump administration investigations

Democrats on the the House Judiciary Committee and House Oversight and Government Reform Committee are already eyeing a handful of issues to highlight and pursue in the first 100 days of the new session, which were areas that they sought answers to under Republican control but received no responses.

Rep. Jerry Nadler, D-N.Y., the incoming chairman of the Judiciary Committee, plans to focus on areas including the family separation policy at the border, voting rights, the Justice Department’s refusal to defend Obamacare, hate crimes involving white supremacists and surveillance of minority groups.

The next chairman of Oversight, Rep. Elijah Cummings, D-Md., already fired a warning shot in late December, sending 51 letters to the White House, multiple federal agencies and others requesting full compliance by Jan. 11, 2019 with previous document requests from Republican Oversight Committee members on multiple topics.

“Many of these requests were bipartisan, and some are now more than a year old," he said. "As Democrats prepare to take the reins in Congress, we are insisting — as a basic first step — that the Trump administration and others comply with these Republican requests.”

Rep. Eliot Engel, D-N.Y., the incoming chairman of House Foreign Affairs Committee, said in December that he plans to “hold hearings after the first of the year about all aspects of the Saudi behavior.”

Russia probe

House Democrats are planning to pursue legislation that would protect special counsel Robert Mueller and the investigation he’s leading into Russian interference in the 2016 election, according to Daniel Schwarz, aide to Nadler on the Judiciary Committee. Republicans blocked a Mueller protection bill in the Senate in November that was pushed by both Democrats and some GOP lawmakers.

Even if a Mueller protection bill were to pass the House, it would face an uphill climb in the Senate, where previous attempts have met with resistance from Majority Leader Mitch McConnell, R-Ky.

House Republicans said after their investigation of Russian meddling that there was no evidence of collusion between Trump associates and Russia during the election, Rep. Adam Schiff, D-Calif., has made clear that he still has some questions.

The Senate investigation of Russian interference is still ongoing. Senate Intelligence Committee Chairman Richard Burr, R-N.C., told NBC News earlier this year that his probe would seep into 2019 as the panel is now digging into the issue of collusion, the last section of their five-part investigation.

Burr told Bloomberg News in November that his committee could take as long as six months after finishing their interviews to write and declassify a final report.

The Republican agenda

Senate Republicans will still hold the power to confirm conservative judicial nominees, including Supreme Court picks should another opening arise. They’ll also have to spend the first half of the year holding confirmation hearings for new Cabinet nominees, including William Barr, Trump's attorney general pick. Then there’s the next Interior secretary nominee, who Trump has not yet chosen; Andrew Wheeler, his nominee for EPA chief; and State Department spokeswoman Heather Nauert, who’s been tapped to serve as ambassador to the United Nations.

House Democratic leadership has shown little appetite for launching impeachment action against President Trump — in part because Senate Republican defections are viewed as unlikely. If the House did swing into impeachment action, the Senate GOP would serve as the president's last line of defense.

McConnell has not yet laid out other specific goals for next year, but incoming House Minority Leader Kevin McCarthy, R-Calif., has mentioned a few areas his caucus plans to focus on under a Democratic majority — including making tax cuts permanent, securing military pay raises, battling the opioid epidemic and tackling the national debt — but life in the minority means the only issues on the list getting tackled are ones where both parties' priorities align.