(CNN) Sen. Bernie Sanders is a top contender in the race for the 2020 Democratic presidential nomination, but for most of his career in public life, he was a fierce critic of the party and its standard-bearers.

In one speech in 1989, Sanders said that the Democratic Party could not be turned around and that it was impossible to build a progressive movement within the party. More than two decades later, in 2011, he suggested that the party should change its name to the "Republican-lite" party.

Sanders' derision of the Democratic Party was discussed during his unsuccessful 2016 presidential run. But the little-known1989 speech reviewed by CNN's KFile reveals the depth of Sanders' criticisms of the party he has now pledged to represent if he becomes the Democratic nominee in 2020. And though Sanders has endorsed the Democratic presidential candidate in every election the 1980s, including campaigning to elect Barack Obama and Hillary Clinton, his past criticism of Democrats has fueled some of the continued skepticism of his candidacy from the more establishment wing of the party.

But his comments also highlight how much the Democratic Party has shifted to the left in recent years -- and the central role Sanders himself has played in that transformation.

Before seeking the 2016 Democratic presidential nomination, Sanders said he would not run as an independent, saying he refused to be "a spoiler" who would elect a Republican. When he was seeking the Democratic presidential nomination that year, he told NBC's Meet the Press he no longer believed politics had to be changed from outside the Democratic Party. And, his stronger-than-expected challenge to Hillary Clinton showed that an insurgent, progressive movement could drive the conversation around policy and politics within the party.

"There was a rightward drift in the Democratic Party in the '80s and '90s," Jeff Weaver, a senior adviser to the Sanders campaign, told CNN. "The rank and file of the party has rejected" that ideology.

Weaver added that Sanders had been "working within the institution of the Democratic Party since he's come to Washington, DC. I don't think that this is something new for him. I think the critiques that he has leveled are critiques that many Democrats, including other contenders for the nomination, are leveling at this time, as well."

Sanders in the '80s and '90s: The Democratic Party can't be turned around

Sanders leveled some of his most pointed critique at the Democratic Party in remarks he made to the National Committee for Independent Political Action in New York City on June 22, 1989.

"I know that there are people, good and honorable people, people who are friends of mine, who believe that the Democratic Party can be turned around. I don't," Sanders said, according to an edited transcript published in the December edition of Monthly Review, an independent socialist magazine.

"It is absolutely imperative that the progressive movement raise the issues and the analyses which will educate the people of our nation to begin to understand what the hell is going on," he added. "And I honestly don't believe that that can take place within the Democratic Party."

When Jesse Jackson ran for president in 1988, Sanders backed him but said he did not agree with Jackson about working within the party system.

"Is Jesse Jackson's presence good or bad?" Sanders said to The Gadfly, a student newspaper of the University of Vermont, in 1987. "I think you would have to be crazy not to understand that Jackson has had an important impact on the American political scene. I am not a member of the Rainbow Coalition. I am a member of the Progressive Coalition. I do not agree with Jesse Jackson on all of his issues by any means and I disagree with working within the Democratic Party, okay?"

Throughout much of the 1990s, Sanders struck a similar tone when talking about the Democratic Party.

"Now my own view, and it has been my view for many, many years, is that what we need in this country is what Jackson calls a Rainbow Coalition, but it has to be done outside of the Democratic Party," Sanders told a local Democratic Socialists of America group in 1991.

"I think that nationally, the party has on issue after issue sold out so many times that if you go before the people and say, 'Hey, I'm a Democrat,' you don't usually generate a lot of enthusiasm."

In a C-SPAN appearance from 1995, Sanders addressed a caller's concern that the Democratic Party strayed too far from its roots by saying that Democrats are so conservative they would no longer fit into President Harry Truman's Democratic Party.

"Read what Harry Truman said in 1948," Sanders said. "Truman was a moderate Democrat. The progressives were upset about him because he was too conservative. He said things about the class structure of America in 1948 virtually no Democrat will say today. And what has been said often enough ---- you don't need another Republican Party. We need a party to speak for the vast majority of the people whose standard of living is in decline. The gentleman makes a very good point."

Sanders in the 2000s and 2010s: The Democrats should be called 'Republican-lite'

During the 2000 election, Sanders initially declined to say if he would support Vice President Al Gore. In May 2000, he introduced Ralph Nader at an event during the candidate's swing through Vermont when he was seeking the Green Party nomination.

"He's an old-fashioned guy who believes that maybe the ordinary people should be running this country rather than the multinational corporations," Sanders said as he introduced Nader at an event in Montpelier, according to the Associated Press

In August of that year, Sanders called the Democratic ticket the most conservative put forth by Democrats in many years.

"I think it's fair to say that there is great dissatisfaction on the part of progressives," Sanders said, according to the Associated Press . "The Democratic ticket is probably the most conservative ticket that the Democrats have brought forth in many years."

Still, Sanders declined to say if he was supporting Gore or Green Party candidate Ralph Nader in the race, saying he feared George W. Bush could get elected to the White House.

"Is that the right decision or is it the right decision to vote for the ideologically weaker candidate, in terms of Gore, but make sure that a guy like George Bush does not get elected president and we don't run the possibility of Republican control of the House, the Senate and the White House," Sanders said.

In his Senate races in 2006, 2012 and 2018, Sanders ran for the Democratic nomination in Vermont, but declined it after winning to run as an independent without a Democratic Party -- a move supported by the Vermont Democratic Party . Still, he maintained his sharp criticism of the party, attacking the party over any cuts to social programs and saying he might encourage a primary challenge to President Obama in 2012.

In November 2011, Sanders criticized Democrats for their response to the Budget Reform Act of 2011, saying the super committee to develop a deficit reduction plan shouldn't look at cuts in social security or Medicare.

"My suggestion was literally to the Democratic leadership, simply change the name of the party from the Democratic Party to the Republican-lite versus Republicans and say, 'Yeah, we're bad, but we're not as bad as these guys,'" said Sanders on the Thom Hartmann Program.

Speaking with WNYC radio in January 2013, shortly after Obama's reelection, Sanders said he didn't believe that Obama was a progressive.

"No. Oh, I think he's a pretty honest guy and the president, as you may recall, just a few weeks ago said that if we were in the 1980s, he would have been considered something like a moderate Republican," Sanders said. "And I think he's kind of a centrist somewhere in the middle of the Democratic Party. But no, I don't think he is a progressive."

"I was very disappointed in terms of his unwillingness to be more aggressive in standing up to what has increasingly become a right-wing extremist Republican Party," Sanders said. "And he really hasn't, to my mind, done that effectively."