Lawmakers seeking special counsel Robert Mueller's report are likely to face a protracted legal battle that will turn on U.S. President Donald Trump's right to keep communications with his advisers private, legal and political experts said.

On Sunday, U.S. Attorney General William Barr sent a summary to lawmakers saying the Mueller investigation found Trump's 2016 presidential campaign did not conspire with Russia. But the probe left unresolved the question of whether Trump engaged in obstruction of justice, setting out "evidence on both sides of the question."

Barr concluded there was insufficient evidence to bring an obstruction case against Trump, prompting Democratic lawmakers to call for the release of the full report and the underlying evidence Mueller relied on.