(CNN) Three presumptive Democratic committee chairman on Thursday outlined on a call for Democratic lawmakers the steps they're planning to take to investigate the firing of Attorney General Jeff Sessions and protect special counsel Robert Mueller's investigation following the appointment of acting Attorney General Matt Whitaker.

On the call for current and new Democrats, organized by Minority Leader Nancy Pelosi, the likely incoming Judiciary Chairman Jerry Nadler of New York said it was a "crisis moment" they had to address, according to a source familiar with the call.

Two sources said that Nadler, presumptive Intelligence Chairman Adam Schiff of California and presumptive Oversight Chairman Elijah Cummings of Maryland spoke to explain the steps they had already taken since Whitaker's appointment, including directing numerous agencies to preserve documents in the Mueller investigation and related to Sessions' firing.

The letters, which signaled that Democrats plan to investigate the ousting when they're in the majority next year, were the "first move" to protect Mueller from being fired, Schiff told CNN's Jake Tapper on Thursday

One source on the call said the Democratic speakers laid out the case for rank-and-file lawmakers so they "all understood in common terms how grave of a mistake" it was for Trump to have fired Sessions.

Read More