Trump officials say they feel especially bullish about one key argument against calling additional impeachment witnesses: It could compromise America's national security.

The big picture: People close to the president say their most compelling argument to persuade nervous Republican senators to vote against calling new witnesses is the claim that they're protecting national security.

Why it matters: They're banking on it to speed up the trial, according to people close to the president.

What we're hearing: White House counsel Pat Cipollone, who considers himself a civil libertarian, is expected to argue that the obstruction of Congress article is dangerous and could forever undermine the power of the executive office to protect privileged information.

Cipollone will likely frame the Senate trial as a defining moment to set the precedent for executive privilege, especially on national security matters, per a source familiar with his thinking.

This approach is something Cipollone is particularly proud of, and one that he is happy to test in court, the source said.

The argument: Presidential claims of executive privilege are especially strong when they involve conversations about national security.

Weakening that privilege would make presidents less candid when they seek counsel from their advisers on national security (think John Bolton).

The bottom line: Sources close to Trump's legal team have privately expressed confidence that former national security adviser Bolton will ultimately honor Trump's assertion of executive privilege.

Go deeper: Trump's concede-nothing defense