The Republican Party is in danger of splitting apart, former Clinton advisor Sidney Blumenthal said Thursday.
"I think that the Republican Party is in danger of going the way of the Whigs," he said, referring to the 19th century political party whose members split to form today's Republican Party and the defunct Know Nothing Party.
"Parties do break up. They do disintegrate. They disappear, and they become something else," he told CNBC's "Squawk Box".
Presumptive Republican presidential candidate Donald Trump is meeting with GOP House Speaker Paul Ryan on Thursday following an exchange last week in which Ryan said he was not yet ready to support Trump. Trump fired back that he was not ready to support Ryan's agenda.
Blumenthal, who advised both Bill and Hillary Clinton, is currently promoting his book, "A Self-Made Man: The Political Life of Abraham Lincoln." Blumenthal has also worked for MediaMatters, which seeks to counter conservative messaging in the media.
Blumenthal said the Whig Party broke up for the same reason divisions are emerging in the Republican Party. The party was riven apart by disagreements over race and immigration, he said.
The Southern U.S.-centered Know Nothing members were nativists who objected to a wave of immigration, while Abraham Lincoln was anti-nativist, he said. In order to set up a new party, Lincoln had to figure out how to deal with a movement of radical abolitionists, Blumenthal added.
"So you had to deal with the party people and the movement people, so it all sounds similar, not only on the Republican side, but also on the Democratic side today," he said.
The Democrats face their own insurgence in the form of Bernie Sanders supporters, who have embraced positions further to the left than front-runner and establishment candidate Hillary Clinton has taken.
Though the delegate math is against Sanders, Blumenthal said the Vermont senator has the right to go head-to-head with Clinton until the very end, just as Clinton did against Barack Obama in 2008.
He said campaigning to the end gives Sanders a platform to push his agenda at the Democratic National Convention, but cautioned that Sanders must consider his relationship with a potential Clinton White House.
Blumenthal worked for the Clinton Global Initiative after the Obama administration reportedly blocked his hiring at the State Department. During that time, he passed intelligence about Libya to then-Secretary of State Clinton. Those correspondences came under scrutiny after The New York Times reported the intelligence was sourced from business associates Blumenthal advised and who sought contracts from Libya's transitional government.
On Thursday, Blumenthal said he made no money from the associates and noted he has called for his comments before a congressional committee on the 2012 Benghazi attack to be made public.
He declined to comment on an FBI investigation into Hillary Clinton's use of a private email server to conduct State Department business, which has shined a spotlight on some of his Libya correspondences with Clinton. On Wednesday, he told CNN that investigators would find no bombshell that could derail her campaign.