Larry Tribe, professor of Constitutional law at Harvard Law School, says that as long as Pelosi is opposed to the State of the Union happening, as scheduled, on Jan. 29, it won’t happen.

“Pelosi was basically uninviting the President, but doing it diplomatically,” Tribe tells Fortune. “The form in which the State of the Union address is delivered isn’t constitutionally specified, but if it’s to be delivered as a Joint Session of Congress as has become the custom, the House and Senate must jointly adopt a resolution to schedule it. Without that, he can’t come. Neither chamber has adopted such a resolution yet, and as long as Pelosi opposes it, it’s DOA.”

In a letter to Trump Monday, Pelosi cited “security concerns” due to the partial government shutdown and asked Trump to either not deliver the address until the government had reopened or to deliver it in writing on Jan. 29, as was done regularly until the presidency of Woodrow Wilson in 1913.

While the Constitution requires the president deliver a State of the Union address, there’s no language in there that demands it be a speech or in person.

“Since the start of modern budgeting in Fiscal Year 1977, a State of the Union address has never been delivered during a government shutdown,” Pelosi wrote. “The extraordinary demands presented by [the security needs of the speech] require weeks of detailed planning with dozens of agencies working together to prepare for the safety of all participants. … However, both the U.S. Secret Service and the Department of Homeland Security have not been funded for 26 days now—with critical departments hamstrung by furloughs.”

The government shutdown is already the longest on record and shows no signs of stopping, even as the effects begin to ripple through the country in the form ofdelayed tax refunds, airport delays, and possibly worse in the future. Trump is demanding $5 billion to fund a wall along the Mexican border. Democrats are refusing to agree to that.