WASHINGTON If President Donald Trump can get away with blocking Congress from investigating his pressure campaign against Ukraine , Democrats argued Friday, then the presidency would essentially have unlimited power.

We would create a system that allows this president and any future president to really do whatever he or she wants, impeachment manager Rep. Val Demings (D-Fla.) told senators Friday.

Democrats have accused Trump of engaging in a cover-up in addition to abusing his power. When the House investigated Trumps involvement with Ukraine, the administration refused to turn over documents or allow key individuals to testify.

Demings said the administration has blocked more than 70 requests for documents and testimony, but that Democrats have been able to assemble their impeachment articles thanks to more than a dozen courageous public servants who were willing to follow the law, comply with subpoenas and tell the truth.

Democrats are finishing their third and final day of opening arguments Friday, with each impeachment manager taking a few minutes at the top of his or her presentation to thank the increasingly restless senators for listening. Demings spoke to the obstruction-of-Congress charge, the second of two impeachment articles that Democrats have brought against Trump.

The first article charges Trump with abuse of power for his scheme to pressure Ukraine into announcing an investigation of his highest-polling political opponent, former Vice President Joe Biden, a 2020 Democratic presidential hopeful. A series of diplomats testified last fall that the pressure campaign on Ukraine subverted U.S. foreign policy for the presidents personal benefit.

Democrats said their case is overwhelming, but that it would have been even stronger if more witnesses had testified.

Rep. Zoe Lofgren (D-Calif.), another of the seven impeachment managers , who worked as a staffer on the House Judiciary Committee during the Watergate hearings in the 1970s, said President Richard Nixon was more forthcoming with aides and documents than Trump has been.

No president ever used the official power of his office to prevent witnesses from giving testimony before Congress in such a blanket and indiscriminate manner, Lofgren said.

Impeachment manager Rep. Jerry Nadler (D-N.Y.), wrapping up his comments about how Trump had obstructed congressional investigations, said the president was the first and only president ever to declare himself unaccountable.

Only his will goes, Nadler said Friday, urging the Senate to remove Trump from office. He is a dictator. This must not stand.

On Saturday morning, Trumps defense team is expected to start the first of as many as three days of arguments against removing Trump from office, and then the Senate will vote on whether to call witnesses. Its unclear if enough Republicans will break ranks four are needed to vote in favor of gathering more evidence.

Trump attorney Jay Sekulow told reporters on Friday that the idea that you obstruct Congress by exercising constitutional privileges is absurd. He said the courts have said that executive privilege rights are fundamentally important rights to weigh our separation of powers under our form of government.

Rep. Adam Schiff (D-Calif.), who is leading the team of House impeachment managers, said the managers might ask Supreme Court Chief Justice John Roberts to rule on questions of executive privilege although he noted that Senate Republicans might overrule him.

You cannot use executive privilege to hide wrongdoing or criminality or impeachable misconduct, and that is exactly the purpose for which they seek to use it, Schiff said.

Igor Bobic contributed reporting.

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