Neal Katyal, who served as acting solicitor general under President Obama, said Sunday that evidence supports articles of impeachment against President Trump Donald John TrumpBiden says voters should choose who nominates Supreme Court justice Trump, Biden will not shake hands at first debate due to COVID-19 Pelosi: Trump Supreme Court pick 'threatens' Affordable Care Act MORE for bribery, abuse of power and obstruction of justice.

“Here the most important thing is not just Ukraine but the president’s reaction to Ukraine,” Katyal said on CBS’ “Face the Nation,” adding that Trump has engaged in “unprecedented obstruction.”

Former Solicitor General @neal_katyal tells @margbrennan that @realDonaldTrump has engaged in “unprecedented obstruction,” and lays out three potential articles of impeachment. WATCH: pic.twitter.com/KxQLXsgye2 — Face The Nation (@FaceTheNation) December 8, 2019

“No president in our history has ever done this, saying unilaterally ‘I declare the entire investigation illegitimate,” Katyal said, noting that while Richard Nixon considered such a strategy he ultimately backed down. “If this president can do it every president can do it whether it was Obama before, or a President Elizabeth Warren Elizabeth WarrenOvernight Defense: Appeals court revives House lawsuit against military funding for border wall | Dems push for limits on transferring military gear to police | Lawmakers ask for IG probe into Pentagon's use of COVID-19 funds On The Money: Half of states deplete funds for Trump's 0 unemployment expansion | EU appealing ruling in Apple tax case | House Democrats include more aid for airlines in coronavirus package Warren, Khanna request IG investigation into Pentagon's use of coronavirus funds MORE or Bernie Sanders Bernie SandersTrump, Biden will not shake hands at first debate due to COVID-19 Sanders tells Maher 'there will be a number of plans' to remove Trump if he loses Sirota reacts to report of harassment, doxing by Harris supporters MORE.”

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“Impeachment is so central to our checks and balances, what he’s doing is a fundamental threat to our Constitution,” Katyal added.

Meanwhile, Katyal said, the quid pro quo that several House witnesses have testified was in place in which Trump allegedly attempted to leverage military aid to Ukraine for an investigation into former Vice President Joe Biden Joe BidenBiden says voters should choose who nominates Supreme Court justice Trump, Biden will not shake hands at first debate due to COVID-19 Joe Biden should enact critical government reforms if he wins MORE’s son Hunter Biden could serve as an article of impeachment for bribery.

Katyal said impeachment proceedings should have “a narrow focus,” adding “some of the other evidence [from] Mueller can come in as kind of supporting evidence, that’s how the Nixon impeachment had it, the articles of impeachment there focused on Watergate but then it brought in some other things.”