House Intelligence Committee Chairman Adam Schiff rejected the GOP’s request that the panel summon Vice President Joe Biden’s son, Hunter Biden, to testify at public impeachment hearings next week.

The California Democrat also signaled he will not agree to some of the other witnesses Republicans requested Saturday, including Alexandra Chalupa, a former Democratic National Committee staffer who Republicans said worked with the Ukrainian Embassy in Washington, D.C., “to try to get political dirt on then-candidate Trump’s campaign.”

Schiff, who will head public impeachment hearings beginning Wednesday, said in a Saturday statement that he will consider a list of witness requests provided by the GOP.

“This inquiry is not, and will not serve, however, as a vehicle to undertake the same sham investigations into the Bidens or 2016 that the President pressed Ukraine to conduct for his personal political benefit, or to facilitate the President’s effort to threaten, intimidate, and retaliate against the whistleblower who courageously raised the initial alarm," he said.

Democrats plan to open the public hearings on Wednesday with testimony from acting Ukraine Ambassador Bill Taylor, who said in a closed-door deposition that he believed President Trump was withholding critical security aid from Ukraine in order to get Ukrainian government officials to investigate Biden and the Democrats.

Democrats are building an impeachment case that accuses Trump of abuse of power and obstruction of Congress.

California Rep. Devin Nunes, who is the top Republican on the intelligence panel, sent a witness list to Schiff that included Hunter Biden as well as the anonymous whistleblower, whose complaint about a July 25 phone call between President Trump and Ukraine President Volodymyr Zelensky launched the impeachment proceedings.

[Also read: Republicans seek public testimony of Hunter Biden and anonymous whistleblower at impeachment hearings]

Trump asked Zelensky on the call to investigate former Vice President Joe Biden’s effort to oust a Ukraine prosecutor who was targeting the Burisma Holdings gas company, which employed Hunter Biden for a hefty salary.

Trump also wanted Zelensky to investigate allegations the Democratic National Committee was working with Ukraine officials in 2016 to find information that would be damaging to the Trump presidential campaign.

Republicans want to showcase witnesses who would back up Trump’s concerns about Democratic election interference and Ukrainian corruption and, in particular, why Hunter Biden won a seat on the gas company board despite having no experience.

The GOP list includes Ambassador Kurt Volker, who served as a special envoy to Ukraine. Nunes has requested Volker to serve on the same panel as Taylor.

Republicans also want to hear from David Hale, the undersecretary of state for political affairs, and Tim Morrison, senior director for European and Russian Affairs on the National Security Council.

Morrison said in closed-door testimony that he did not believe Trump did anything wrong in his July 25 conversation with Zelensky.

Schiff is more likely to allow the GOP to call Volker, Hale, and Morrison.