WASHINGTON (AP) — Speaker Mike Johnson will delay sending the House’s articles of impeachment against Homeland Security Secretary Alejandro Mayorkas to the Senate this week as planned, after Republican senators requested more time Tuesday to build a case for a full trial.

The sudden change of plans cast fresh doubts on the proceedings, the historic first impeachment of a Cabinet secretary in roughly 150 years. House Republicans impeached Mayorkas over the Biden administration’s handling of security and immigration at the U.S.-Mexico border.

Senators were expected to swiftly dismiss the House-passed charges against Mayorkas. Democrats, who hold majority control of the chamber, argue the charges do not rise to the constitution’s bar of high crimes and misdemeanors.

Johnson was planning to have the House impeachment managers transmit the articles against Mayorkas on Wednesday evening. Under procedural rules, that would require senators to convene the next day as jurors for a trial to decide whether to convict or acquit the secretary of charges.

Thursday’s trial was expected to be over quickly after some procedural votes to table or dismiss the charges.

But Republicans intent on holding Mayorkas accountable for border security are pushing for a full trial. Republican senators spoke during a private GOP lunch Tuesday about using a delay to build the case.

“To ensure the Senate has adequate time to perform its constitutional duty, the House will transmit the articles of impeachment to the Senate next week,” said Johnson’s spokesman, Taylor Haulsee. “There is no reason whatsoever for the Senate to abdicate its responsibility to hold an impeachment trial.”