Thursday is shaping up to be a milestone in the impeachment inquiry into President Donald Trump.

A resolution vote will be the first of the full House under the formal impeachment inquiry and will put moderate lawmakers from both parties under scrutiny heading into the 2020 election.

The inquiry is attempting to determine whether the president withheld military aide from Ukraine in exchange for an investigation into Trump's domestic political rivals.

Trump has acknowledged asking Ukraine's Volodymyr Zelensky to look into former Vice President Joe Biden and his son, Hunter Biden, as well as the origins of a 2016 hacking of the Democratic National Committee.

Here is what to know about Thursday's happenings.

What is the House voting on?

The House of Representatives is expected to debate and vote on a resolution offering rules for hearings on the impeachment inquiry, and also outlines powers for Republican members to issue subpoenas.

It's not the vote to impeach the president. Rather, it formalizes the impeachment process and establishes rules of conduct going forward.

So far, the House Democrat-led impeachment proceedings have taken place in private, prompting outrage from Republicans who decry the "secrecy" of the process and claim they are being purposefully kept in the dark, despite that several dozen House Republicans are authorized to participate.

Why the vote is important

The resolution formalizes the public phase of the investigation with hearings and evidence sharing with the president’s counsel, even as Republicans continue to criticize the process as a “sham.”

The resolution establishes the ability for the White House to call and cross-examine witnesses in the phase-controlled by the Judiciary Committee, subject to approval from those in charge.

More:Republicans say Trump impeachment probe is happening in 'secret,' but 47 GOP lawmakers have access

"The American people will hear firsthand about the president's misconduct," Intelligence Committee Chairman Adam Schiff, D-Calif., said of the resolution.

The White House says that the impeachment process is an "illegitimate sham" despite the upcoming House vote. Republicans in Congress have urged their House colleagues to vote against the measure. Lawmakers have not specified what time on Thursday the resolution will be discussed.

Subpoena battle lands in court

A federal judge scheduled a hearing for Thursday afternoon after Charles Kupperman, a former deputy national security adviser, defied a House subpoena and asked the court to decide whether he would need to testify.

Kupperman filed suit last week and missed his deposition date on Monday. He said the judicial branch should decide between the legislative branch (the House through its subpoena) or the executive branch (the White House, which directed him not to comply).

If he testifies, he is expected to be asked by lawmakers about the aid money that was delayed to Ukraine after being approved by Congress.

Who is the next witness?

The National Security Council senior director for Russian affairs, Timothy Morrison, is scheduled for a deposition Thursday.

Morrison could provide more detail about what Bill Taylor, the top U.S. diplomat in Ukraine, told the House inquiry last week.

Taylor said Morrison told him during an August phone call that the “president doesn’t want to provide any assistance at all,” referring to the nearly $400 million in military aid.

More:Key takeaways from Ukraine diplomat Bill Taylor's 'explosive' opening statement

On Tuesday, another NSC official gave testimony. Lt. Col. Alexander Vindman, the director of European affairs, testified that he had raised concerns about the call on more than one occasion.

Contributing: Bart Jansen