WASHINGTON (AP) — Senators took oaths Tuesday to ensure “impartial justice” as jurors in Donald Trump's historic impeachment trial, proceedings that will test Republican loyalty to the former president and the power he still wields after the deadly siege at the U.S. Capitol.

But some Republican senators are challenging the legitimacy of the trial and questioning whether Trump's repeated demands to overturn Joe Biden’s election really constitute “incitement of insurrection” in the Jan. 6 riot.

Republican Sen. Rand Paul of Kentucky said he plans to force a vote on whether the impeachment trial of a former president is allowed under the Constitution.

“A sham, this is, a travesty, a dark blot on the history of our country,” Paul said in a fiery speech on the Senate floor.

He called on Senate colleagues to stop this “kangaroo court” and compared the way Trump egged on the crowd outside the White House before the mob stormed the Capitol to past speeches by Democratic lawmakers, including now-Vice President Kamala Harris.

What seemed for some Democrats like an open-and-shut case that played out for the world on live television — as Trump encouraged a rally mob to “fight like hell" for his presidency — is running into a Republican Party that feels very differently. Not only are there legal concerns, but senators are wary of crossing the former president and his legions of followers. Security remains tight at the Capitol.

On Monday, the nine House Democrats prosecuting the case against Trump carried the sole impeachment charge of “incitement of insurrection” across the Capitol in a solemn and ceremonial march along the same halls the rioters ransacked three weeks ago.

In a scene reminiscent of just last year — Trump is the first president twice impeached — the lead House prosecutor, Rep. Jamie Raskin of Maryland, stood before the Senate to read the House resolution charging “high crimes and misdemeanors.”

But Republican denunciations of Trump have cooled since the riot.

Sen. John Cornyn, R-Texas, asked if Congress starts holding impeachment trials of former officials, what's next: “Could we go back and try President Obama?”

Besides, he suggested, Trump has already been held to account. “One way in our system you get punished is losing an election.”

Arguments in the Senate trial will begin the week of Feb. 8, and the case against Trump, the first former president to face impeachment trial, will test a political party still sorting itself out for the post-Trump era.

For Democrats the tone, tenor and length of the trial so early in Biden's presidency poses its own challenge, forcing them to strike a balance between their vow to hold Trump accountable and their eagerness to deliver on the new administration's priorities following their sweep of control of the House, Senate and White House.

Chief Justice John Roberts is not expected to preside at the trial, as he did during Trump’s first impeachment, potentially affecting the gravitas of the proceedings. The shift is said to be in keeping with protocol because Trump is no longer in office.

Instead, Sen. Patrick Leahy, D- Vt., who serves in the largely ceremonial role of Senate president pro tempore, is set to preside.

Leaders in both parties agreed to a short delay in the proceedings that serves their political and practical interests, even as National Guard troops remain at the Capitol amid security threats on lawmakers ahead of the trial.

The start date gives Trump’s new legal team time to prepare its case, while also providing more than a month's distance from the passions of the bloody riot. For the Democratic-led Senate, the intervening weeks provide prime time to confirm some of Biden’s key Cabinet nominees.

An early vote to dismiss the trial probably would not succeed, given that Democrats now control the Senate. The House approved the charge against Trump on Jan. 13, with 10 Republicans joining the Democrats.

Mounting Republican opposition to the proceedings indicates that many GOP senators will eventually vote to acquit Trump. Democrats would need the support of 17 Republicans — a high bar — to convict him.

As Republicans say the trial is not legitimate, Democrats reject that argument, pointing to an 1876 impeachment of a secretary of war who had already resigned and to opinions by many legal scholars.

Democrats also say that a reckoning of the first invasion of the Capitol since the War of 1812, perpetrated by rioters egged on by a president as Electoral College votes were being tallied, is necessary.

Senate Majority Leader Chuck Schumer said failing to conduct the trial would amount to a “get-out-jail-free card” for others accused of wrongdoing on their way out the door. He said there’s only one question “senators of both parties will have to answer before God and their own conscience: Is former President Trump guilty of inciting an insurrection against the United States?”

A few GOP senators have agreed with Democrats, though not close to the number that will be needed to convict Trump.

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Associated Press writer Hope Yen contributed to this report.