The US Congress will return from its summer recess on Tuesday for a supercharged September sitting that will be a major test of the Republican Party’s capacity to govern the United States.

September is always budget season in Congress, with the federal government’s financial year beginning on 1 October. This September’s legislative manoeuvring will be far more consequential than usual: the United States faces a debt ceiling breach, a government shutdown, or both if Congress does not get its act together in the 12 days it is in session next month.

To avoid a government shutdown, the House and Senate must pass – and then the president must sign – a budget or at least a ‘continuing resolution’ (CR) that kicks the can down the road for a few months. That is a tall order in a tight timeframe. House Speaker Paul Ryan and Senate Majority Leader Mitch McConnell will have to woo most of their Republican troops in order to pass a budget or a CR.

The budget, it seems, will have to include funding for the wall with Mexico to be signed by President Trump. At a rally in Phoenix last week, Trump indicated that he was prepared to shut down the government if the budget does not include sufficient funds to build the wall. The last government shutdown in 2013 lasted 16 days, with most of the 2 million person federal workforce barred from working and national parks closed. Moody’s Analytics estimated the economic impact of it was US$23 billion.

Debt ceiling negotiations are joined at the hip with the budget process. Congress must raise the US$19.8 trillion debt ceiling by 29 September to give the US Treasury the authority and ability to raise funds in order to pay the bills of government. Mitch McConnell said last week there was “zero chance” that Congress would allow a default on federal debt, but a significant bloc of hardline Republicans have vowed to oppose an increase to the debt ceiling that is not offset by spending cuts.

Hurricane Harvey has added another major priority to the agenda. To help address the damage in Texas, Congress will likely try to reauthorise the National Flood Insurance Program, which is currently US$25 billion in debt.

All indications are that September will be a very difficult month for Paul Ryan and Mitch McConnell. Each will have to manage recalcitrant members of their own caucus while simultaneously dealing with a president who does not seem to consider Congress a co-equal branch of government and has been prepared to lash out against sitting Republican representatives and senators, not least McConnell himself.

Expect late-night wrangling on Capitol Hill, serious strains emerging within the governing Republican Party, and a ticking time bomb as Congress tries to pass a budget and raise the debt ceiling.