Washington (CNN) Nine new witnesses in the impeachment inquiry are scheduled to give public testimony in the coming days, the second week of highly anticipated open hearings.

The two public hearings last week were at times boring and breathtaking, though it's not clear that they did much to move the needle of public opinion. They shed new light on President Donald Trump's dealings with Ukraine and how his actions cut against long-established US policy.

Here are five things to look for as another busy week in Washington kicks off.

Witnesses get a platform to rebut Trump's attacks

Some of the upcoming witnesses, chiefly Vindman and Williams, have been attacked by Trump and his allies in the run-up to their scheduled testimony. Many of these attacks came after transcripts of their closed-door testimony were released, showing how both officials had serious concerns about Trump's now-infamous call with Ukrainian President Volodymyr Zelensky.

After Williams' transcript came out Saturday, Trump tweeted about her on Sunday , saying: "Tell Jennifer Williams, whoever that is, to read BOTH transcripts of the presidential calls, & see the just released ststement from Ukraine. Then she should meet with the other Never Trumpers, who I don't know & mostly never even heard of, & work out a better presidential attack!"

There's no evidence Williams or any of the other witnesses are "Never Trumpers," a label that Trump has used in recent weeks to malign anyone who said anything bad about him. Two key witnesses rebutted this allegation last week, and Williams will surely be asked about it too.

Things were more personal with Vindman: Trump's allies questioned his patriotism and resorted to xenophobic insults by pointing out that Vindman was born in Soviet-era Ukraine and suggesting that he had been working on behalf of Ukraine during his White House tenure.

It seems unlikely that Vindman, an Army officer who received a Purple Heart during his service in Iraq, will let these baseless attacks remain in the ether without responding. We'll find out Tuesday just how forcefully he wants to rebut the attacks from Trump, who is his boss after all.

Pence's turn in the spotlight

Williams' expected testimony on Tuesday will pull Pence back into the impeachment spotlight. Pence has staunchly defended Trump's actions throughout the affair, though there are still lingering questions about how much he knew about the quest for Ukraine to launch investigations into former Vice President Joe Biden and Trump's other political rivals.

In her private deposition , Williams described Pence's conversations with Zelensky, which took place while Trump and a group of trusted diplomats were pressing for the new investigations. She also mentioned that Zelensky asked Pence about the holdup of $391 million in US military assistance for Ukraine during their September 1 meeting on the sidelines of an event in Poland.

House Democrats asked Pence six weeks ago to hand over documents about his Ukraine dealings, though the deadline came and went without any action by Pence's office. So far, Democrats haven't subpoenaed Pence, though they subpoenaed the White House and several other senior Trump administration officials to provide documents and testify in the inquiry.

It's possible that Williams' public hearing could raise the stakes for Pence. But it could also give Williams a chance to insulate Pence from the irregular channel that Trump's lawyer Rudy Giuliani and others in the administration used to circumvent official policy toward Ukraine.

Firsthand accounts of the Trump-Zelensky call

Key players from both parties agree that Trump's July 25 phone call with Zelensky might just be the most important evidence in the impeachment inquiry. Some Democrats say the transcript contains incontrovertible proof that Trump abused his powers. Republicans point out that Trump never explicitly threatened to withhold US military aid. Trump claims it was a "perfect" call.

On Tuesday, the American people will hear for the first time from three White House officials who were actually listening to the conversation in real-time: Vindman, Williams and Morrison.

They've already shared their story behind closed doors. Vindman and Williams had concerns with the conversation. Vindman and Morrison understood from the call that Zelensky wasn't going to get the much-needed military aid until he did what Trump wanted. It'll be critical to see if these witnesses say -- with the public watching -- that there was a "quid pro quo."

Walls are possibly closing in on Sondland

The pressure has been steadily mounting over the past few weeks for Gordon Sondland , the current US ambassador to the EU, who definitely has the most at stake when he testifies on Wednesday, and what the four witnesses says on Tuesday about him could be another factor.

Some Democrats have already accused Sondland of lying in his closed-door testimony, and analysts are raising the possibility that he might refuse to answer some questions and cite his Fifth Amendment rights against self-incrimination.

That's because he's now in a tough spot. He already had to revise his testimony and admit that he told a senior Ukrainian official that they probably wouldn't get the military aid until they publicly announced the investigations that Trump wanted. And new testimony last week revealed a previously unknown conversation between Sondland and Trump about the probes.

All four witnesses testifying Tuesday mentioned Sondland in their closed-door depositions, and the picture they paint Tuesday in their testimony could shade the line of questioning for the EU ambassador the following day.

Right now, Sondland and his lawyers are probably trying to navigate the situation in a way that keeps him out of legal trouble but also doesn't anger the President, all while answering questions truthfully and completely. Anything less could have some dire consequences.

Can the GOP undermine the case against Trump?

Besides complaining about the process, which they say is unfair, House Republicans have been trying to blow some holes in the Democrats' growing case against Trump.

They think they'll get a boost Tuesday afternoon when Volker testifies. For one, he previously testified that he wasn't aware of any "quid pro quo" with Ukraine. That gives the GOP a strong talking point, because Volker was supposed to be Trump's point person for all things Ukraine.

Volker also said that everything he did was meant to further true US interests in Ukraine, even including his conversations with Giuliani. Several other witnesses have said that Giuliani's role in the Ukraine affair was alarming and undermined US government policy toward Ukraine.