If you want to understand the situation facing Congress in September, imagine resolving the thorniest problem you can think of in the space of one month.

Now multiply that task by four and add President Trump.

This is what awaits lawmakers as they return from summer break this week. In the small number of working days between now and the end of the month, Congress faces the following decisions: passing a bill to avert a U.S. debt default, renewing government funding to avoid a partial shutdown, reauthorizing critical programs including the Federal Aviation Administration, extending funds for health insurance for about 9 million children and agreeing on emergency aid for victims of Hurricane Harvey.

And that's all while trying to anticipate the behavior of an unpredictable president.

That list isn't even a complete account of all the business on lawmakers' agenda.

Trump has said he wants members to start working on tax cuts. There's a chance Congress will respond if Trump phases out protections for undocumented immigrants brought to the United States as children, as he is expected to do. Lawmakers are under pressure to fund Obamacare cost-sharing reduction payments before Sept. 27, when insurers have to commit to offering plans on the exchanges next year. The Senate needs to pass a defense authorization bill. Committees are expected to interview members of Trump's inner circle about Russia. Depending on how Hurricane Irma evolves, Capitol Hill could find itself responding to yet another destructive storm.

Lawmakers are returning to a jam-packed legislative schedule in September that requires funding the government and raising the debt ceiling. (Oliver Contreras/For The Washington Post)

None of these tasks are straightforward, no matter how common-sense it might seem to fund the government or approve money for disaster relief.

Remember, Congress is beset by historic levels of polarization. Republicans and Democrats can't agree. Republicans and Republicans can't agree. Although the GOP holds majorities in both chambers, the power wielded by conservative hard-liners means party leaders are always at risk of losing control of their agenda.

There are sleeper issues that could imperil any agreement. And lawmakers already view their votes through the lens of next year's elections. They know the people most likely to come to the polls demand their party get the better end of the deal.

Congress hasn't experienced this much pressure since the 2013 "fiscal cliff." It's no wonder Washington is bracing for a long and complicated month.

Here's a rundown of what lawmakers have to do and when.

Sept. 29: The United States hits the debt limit.

Treasury Secretary Steven Mnuchin has named the second-to-last day of September as the final deadline for Congress to raise the debt limit before the United States risks default. That makes it the first deadline lawmakers will face upon returning from August recess.

The debt limit, also known as the debt ceiling, is a legal cap on how much Treasury is allowed to borrow to pay its bills. Congress has to raise it periodically to allow the government to meet its financial obligations. If the Treasury Department falls behind, the stock market could crash and interest rates could surge.

In recent years, debt-ceiling hikes have become high-stakes battles as conservatives demand that spending cuts accompany each increase. This time, it's possible the debt ceiling increase will be linked to aid for Hurricane Harvey, putting conservatives at risk of looking insensitive to victims if they vote no. (An influential conservative lawmaker has already requested that the two issues remain separate.)

Sept. 30: The government runs out of money.

If Trump doesn't sign a funding bill by 11:59 p.m. on the last day of September, the government will partially shut down. Next to raising the debt ceiling, getting a bill to his desk is one of the biggest tasks Congress faces this month.

Lawmakers have several options for handling this. They could approve funding for the government in the short-term. They could approve funding for a year. Or they could mix the two, giving some agencies a year's worth of funding and some only temporary funding.

In this debate, you're likely to hear the term "continuing resolution," or CR. That refers to a spending bill that maintains government funding at the same level for a certain period of time. Lawmakers could resort to a CR as a last-ditch effort to keep the government open. We're likely to see a spending bill that continues funding at existing levels from October until sometime in December.

Last month, Trump said he would not approve a government spending bill unless it included $1.6 billion for a border wall. But this is unlikely to lead to a shutdown: The White House has indicated to congressional Republicans that the CR need not include wall funds.

Sept. 30: Authorization for the Federal Aviation Administration expires.

Congress has to take two steps to keep government agencies running: authorizing their activities and paying for them. The FAA is up for reauthorization this month because its current extension will expire Sept. 30. Debate has centered on a House plan, endorsed by Trump, that would privatize the air traffic control system. The Senate disregarded that push in its legislation.

Sept. 30: Authorization for the National Flood Insurance Program expires.

The National Flood Insurance Program, or NFIP, underwrites the vast majority of U.S. flood policies and was last extended in 2012. Its possible expiration is getting greater-than-usual attention this month given the vast destruction wrought by Hurricane Harvey.

If the program is allowed to expire, it would be unable to write new policies or renew existing ones. And while lawmakers agree it should be reauthorized, many believe it needs reform. The NFIP is approaching its $30 billion borrowing limit with the U.S. Treasury, and claims from Hurricane Harvey are expected to push it over that threshold.

Sept. 30: Funding for the Children's Health Insurance Program expires.

The Children's Health Insurance Program, or CHIP, is a partnership between the federal government and state governments to provide medical insurance for nearly 9 million low-income children. It has traditionally received support from both parties, although the upcoming funding debate become controversial if Republicans seek to attach Obamacare-related provisions they were unable to pass during their health-care effort earlier this year.

No deadline: Hurricane Harvey aid needed.

Lawmakers are already preparing a package of aid for victims of Hurricane Harvey, which has driven more than 1 million people from their homes. The White House is requesting an initial $5.9 billion as a "down payment" for recovery efforts, and the House will take up a short-term aid bill as its first order of business on Wednesday.