Even if congressional leaders and top White House officials reach an agreement on new fiscal 2018 spending levels this week, lawmakers will still need to pass another temporary spending bill to keep the government open past Jan. 19.

Appropriators cannot rewrite all 12 appropriations bills and package them for floor votes before the third stopgap spending bill of fiscal 2018 expires in less than three weeks, aides in both parties said Tuesday.

That means federal agencies and employees are eyeing at least one more temporary funding measure for fiscal 2018, which began Oct. 1. Getting new topline spending levels, which both parties believe is critical given austere totals imposed by the 2011 deficit-cutting law, is just the start.

If those new numbers are solidified this week, the House and Senate Appropriations committees will need to agree on a framework for the 12 spending bills, and then subcommittees will need to decide how to divide their allocations among different departments and agencies.

That takes time — as well as bipartisan, bicameral agreement on where the additional money should go. A senior Democratic aide said so far there “have been zero substantive discussions about policy or numbers within the bills.” The aide also said Democrats aren’t sure what to expect from a Wednesday meeting of the “big four” Hill leaders and two of President Donald Trump’s top aides, given the president’s “unpredictability.”

Trump himself won’t be at the meeting with Senate Majority Leader Mitch McConnell, Senate Minority Leader Charles E. Schumer, Speaker Paul D. Ryan and House Minority Leader Nancy Pelosi.

Instead, Office of Management and Budget Director Mick Mulvaney and White House Legislative Affairs Director Marc Short will represent the administration at the meeting, which is taking place at the Capitol.

White House Deputy Press Secretary Lindsay Walters said Tuesday that Mulvaney and Short “look forward to meeting on the Hill tomorrow to discuss budget caps.”

But budget caps won’t be the only thing on congressional leaders’ minds. The length of the new continuing resolution, and its ability to pass both chambers, will be determined by numerous factors.

Both parties are focused on a bipartisan reauthorization of the Children’s Health Insurance Program; a legislative solution for the nearly 800,000 “Dreamers,” or undocumented immigrants brought to the United States illegally when they were children; and a long-term reauthorization of the foreign surveillance law expiring after Jan. 19.

What’s more, a $81 billion disaster aid package, which passed the House but is stuck in the Senate, could be a sticking point for California, Florida and Texas delegation members who believe the House bill shortchanged needs in their states.

Some lawmakers in both parties also want to see more relief for Puerto Rico and the Virgin Islands.

In a “Dear Colleague” letter released Tuesday, Pelosi wrote that she and Schumer will remain adamant that for every dollar defense spending is increased in a potential caps deal, nondefense discretionary spending should also be increased by a dollar. “In these talks, Leader Schumer and I will continue to insist on parity in the CAPS,” the letter states.

White House Press Secretary Sarah Huckabee Sanders said later Tuesday that Trump “wants a two-year budget deal that provides realistic budget caps and provides certainty for our national security. That’s our biggest and number one priority and that will be the focus, front and center of the conversations taking place this week.”

Pelosi outlined a number of other funding priorities that should be dealt with as part of the spending talks. “We are fighting for funding for the opioid epidemic, veterans, pensions, disaster relief, National Institutes of Health, Children’s Health Insurance Program and community health centers,” she wrote.

While other legislative riders could move independently of the CR or on a fiscal 2018 omnibus, all hold the ability to throw off the vote count on spending bills, which are popular targets for add-ons because if they don’t pass on time, lawmakers face a partial government shutdown.