WASHINGTON — Republican senators allied with President Trump are increasingly arguing that the Senate should not call witnesses or subpoena documents for his impeachment trial because Mr. Trump has threatened to invoke executive privilege, and a legal fight would take too long to resolve.

But it is far from clear that Mr. Trump has the power to gag or delay a witness who is willing to comply with a subpoena and tell the Senate what he knows about the president’s interactions with Ukraine anyway — as Mr. Trump’s former national security adviser John R. Bolton has said he would do.

Here is an explanation of executive privilege legal issues.

What is executive privilege?

It is a power that presidents can sometimes use to keep information secret.

The Supreme Court has ruled that the Constitution implicitly gives presidents the authority to keep internal communications, especially those involving their close White House aides, secret under certain circumstances. The idea is that if officials fear that Congress might someday gain access to their private communications, it would chill the candor of the advice presidents receive and inhibit their ability to carry out their constitutionally assigned duties.