With the standoff in Congress over healthcare spending now in its fourth week, Trump's Republicans and the opposition Democrats are facing increasing pressure to end a crisis that has crippled public services.

More than 60,000 air traffic controllers and Transportation Security Administration (TSA) officers are working without pay, and the White House warned that increasing absenteeism could mean chaos at check-in lines.  

Trump's spokeswoman Karoline Leavitt told reporters the shutdown was already causing "severe impacts" at airports nationwide.

"If the Democrats continue to keep the government closed, we fear there will be significant flight delays, disruptions and cancellations in major airports across the country this holiday season," she said on Thursday.

Airport workers calling in sick rather than working without pay, leading to significant delays, was a major factor in Trump bringing an end to the 2019 shutdown, the longest in history at 35 days.

In Congress, House Speaker Mike Johnson told a news conference that airport staffing shortages were now the reason for more than 50 percent of delays, a huge increase on the normal statistic of 5 percent.

Some 19,000 flights were held up from Saturday to Monday, he said, warning that this rate was "only going to increase," with airport workers taking on second jobs as Uber drivers or delivering food.

"The longer the shutdown goes on, and as fewer air traffic controllers show up to work, the safety of the American people is thrown further into jeopardy," he said.

Politically toxic

Transportation Secretary Sean Duffy, appearing alongside Johnson, said staff in control towers were voicing deepening anger over the shutdown, which has led to an estimated 1.4 million federal workers going without pay.

"I want them coming to their facilities and controlling the airspace, but they're having to make decisions about how they spend their time, to make sure they put food on their table, feed their kids and support their family members," he said.

With no end to the shutdown in sight, the gridlock is beginning to take a personal toll on lawmakers, who fly out of Washington most weekends to return to their home districts. 

During an earnings call with analysts on Thursday American Airlines CEO Robert Isom said Ronald Reagan Washington National Airport, 8 kilometers from the capital, had suffered "operating delays and issues with air traffic control."

After weeks of failed daily votes on a House-passed resolution to reopen the government, the Senate also rejected a bill on Thursday to guarantee pay for troops and some federal employees who have been working for nothing. 

Republicans had hoped that a blockade on troop pay would be seen by some Democrats as politically toxic and might be a catalyst to finally break the party's united stance on the shutdown. 

All but three Democrats voted against the bill, however, arguing that it would have given Trump too much sway over who gets paid and who does not, while offering no help for 750,000 workers placed on enforced leave without pay. 

Democrats say the only path to reopening the government is a Trump-led negotiation over their demands to extend subsidies that make health insurance affordable for millions of Americans, the key sticking point in the standoff.

But Trump has insisted he will not negotiate with Democrats until the shutdown is over.