A nervous Europe awaits Rubio’s speech in Munich.

The congressional delegation to Europe’s biggest annual security summit had been expected to be the largest ever this year, with around 50 lawmakers planning to travel to Munich to reassure allies that the United States could still be counted on as a reliable security and trade partner.But amid a funding battle that is expected to shutter the Department of Homeland Security this weekend, Mike Johnson, the Speaker of the House, abruptly canceled the official delegation of dozens of House members to the Munich Security Conference, which began on Friday.It is standard operating procedure to call off congressional travel during a government shutdown, and a senior House Republican leadership aide said that was the reason for the cancellation.But top Democrats warned that canceling the delegation was the wrong decision in a moment of frayed trans-Atlantic relations.Last year, Vice President JD Vance’s speech at the conference left foreign leaders reeling. And in recent months, President Trump’s threats to invade Greenland and tariff any NATO ally that tried to stand in his way rattled European allies.“It’s more important than ever that we continue to engage with our partners and allies on critically important national security issues, and send a message of strength through unity,” Representative Adam Smith of Washington, the top Democrat on the House Armed Services Committee, said on Thursday.Mr. Smith, who had planned to attend the Munich gathering, added that lawmakers could have returned to Washington within two days to vote if Republicans and Democrats reached a homeland security funding deal.“Our colleagues in the Senate understand that, and Speaker Johnson should as well,” he said.Several senators departed Capitol Hill on Thursday bound for Germany.Senator Lindsey Graham, Republican of South Carolina, is leading a bipartisan convoy of 11 senators, along with Sheldon Whitehouse, Democrat of Rhode Island. Other senators traveling with them include Lisa Murkowski, Republican of Alaska; Mark Warner, Democrat of Virginia; Chris Coons, Democrat of Delaware; and Richard Blumenthal, Democrat of Connecticut.Another bipartisan delegation, led by Senator Jeanne Shaheen of New Hampshire, flew out on Thursday. Ms. Shaheen, the top Democrat on the Foreign Relations Committee, plans to sit down with President Volodymyr Zelensky of Ukraine, as well as Mark Rutte, the Secretary General of NATO, and the prime ministers of Denmark and Greenland.Senator Thom Tillis, Republican of North Carolina and the co-chairman with Ms. Shaheen of the Senate’s NATO Observer Group, is also set to meet with European leaders in Munich alongside Senators Mark Kelly of Arizona and Chris Murphy of Connecticut, both Democrats.Some Democrats privately suggested that Mr. Johnson’s decision was motivated by a desire to punish them for holding up homeland security funding as they press for guardrails on Mr. Trump’s immigration crackdown.They noted that the Defense and State Departments, which facilitate official overseas travel for members of Congress, are fully funded and at no risk of shutting down, and that the Department of Homeland Security plays no role in congressional delegations.While the delegations are bipartisan, some Republicans made the case on Thursday that Democrats should not be allowed to make the trip, given the spending impasse.Senator Eric Schmitt of Missouri criticized Democratic senators for blocking G.O.P. proposals to fund the department, which includes the Transportation Security Administration, then boarding a government plane bound for the conference.“They’re going to shut that down and then get to fly to Munich on the taxpayer dime and trash President Trump’s foreign policy to Europeans,” Mr. Schmitt said in a video posted to social media. “They should not be going to Europe. This shouldn’t be happening. It’s ridiculous.”Several House members still planned to travel to the conference on their own, including Representative Alexandria Ocasio-Cortez, Democrat of New York. She is expected to use the trip to widen her progressive pitch to foreign policy.Representatives Yassamin Ansari of Arizona, Jason Crow of Colorado, and Sara Jacobs of California, all Democrats, are also traveling to the security summit independently.Ms. Ansari and Ms. Jacobs are scheduled to speak on panels this weekend and were members of the official trip, but scrambled to book commercial flights to Germany after Mr. Johnson called it off.Ms. Ansari said in a statement that she looked forward to discussing with foreign allies how the United States could “rebuild our soft power after an era of chaotic foreign incursions mixed with isolationist lies.”