Julian Zelizer is a professor of history and public affairs at Princeton University, and author, with Kevin Kruse, of the new book " Fault Lines: A History of the United States Since 1974 ." Follow him on Twitter at @julianzelizer. The opinions expressed in this commentary are his own. View more opinion at CNN.

(CNN) Former Vice President Joe Biden officially launched his presidential campaign in Philadelphia on Saturday. Presenting himself as the antidote to President Donald Trump, Biden promised that he would unite and bring the nation together again. "I am running to offer our country -- Democrats, Republicans and independents -- a different path," he said.

The speech picks up on some central themes that have animated Biden's wildly successful rollout. The other day he took some observers by surprise when he predicted that Republicans would have an "epiphany" when Trump left office and would be more willing to work with Democrats. He also stirred the pot when he suggested that the President is an aberration and doesn't reflect the state of the Republican Party.

On paper, Biden's appeal to bipartisanship seems perfect. After all, who is against people from different persuasions getting along? Compromise and negotiation are the lifeblood of a functioning democracy. And, at least for now, the strategy is bringing Biden considerable political success. Polls show that he is staying far ahead of both his primary opponents and Trump in swing states like Pennsylvania.

But the criticism that Biden has faced from progressive Democrats is more than misplaced noise from a Twitter universe that some say is disconnected from the "real world." In fact, the progressive critics on Twitter have been fairly realistic in their assessment of the Republican Party -- even if some politicians don't want to hear it. The frustration with Biden's opening salvo reflects a much deeper critique about the state of American politics and what Democrats will need to do to achieve change.

One of the biggest and most important developments in recent American politics has been the extreme rightward shift of the Republican Party. When Biden talks about the possibility of bipartisanship in the near future, he ignores the extremism that has been front and center in recent years. While some observers still like to talk about extremism on "both sides" of the political aisle, the shift has been much more pronounced on the Republican side of the aisle.

The GOP has become more ideologically homogenous as a party and, as a whole, they have moved in a more dramatic direction. Unlike Democratic leaders, who are constantly trying to find some kind of elusive middle ground, Republican leaders -- House Minority Leader Kevin McCarthy, Senate Majority Leader Mitch McConnell and Trump -- all align neatly with some of the big ticket issues that animate the right, including immigration restriction, economic deregulation, abortion and supply side economics.

And Congressmen Mark Meadows and Jim Jordan, two House Freedom Caucus leaders who used highly aggressive tactics to achieve conservative goals when Obama was in office (such as threatening to send the nation into default over budget disputes), now command one of the most powerful caucuses in the House.

Biden must remember that each time Obama reached out his hand to make some kind of deal, the Republicans took a big bite and gave nothing back in exchange. This rightward drift has only become stronger under Trump.

Like Obama, Biden could talk all he wants about reaching across the aisle, but the reality is that the odds remain extremely low that congressional Republicans would agree to anything if he were in the Oval Office. Biden could propose a major social policy based on conservative ideas -- and he would see it turned into opposition fodder, just as was the case with the Affordable Care Act.

He could take a major Democratic cause, such as climate change, and use conservative mechanisms such as cap-and-trade, a market-based system that offers economic incentives to companies to lower their carbon emissions, to achieve reform. But Republicans will still likely vote no.

Biden's bipartisan promise also raises serious questions about what kind of campaign strategy he will undertake in 2020, should he become the nominee. We will certainly see one of the most vicious and brutal presidential campaigns. While Biden can talk about reaching out to Republicans and independents all he wants, the truth is that he will come under assault from the President, congressional Republicans and the conservative-media complex, who collectively will unleash everything they have to stop him.

And it's not a given that many, if any, Republican voters will be willing to switch sides. Hillary Clinton learned this lesson in 2016 the hard way when Republican states didn't flip to blue, despite predictions that conservative voters would see the light. To be successful, Biden will have to drop some of the rhetoric about moving beyond red and blue in order to make sure that Democrats in states like Ohio are as mobilized and energized as the Republicans are.

Stay up to date... Sign up for our new newsletter. Join us on Twitter and Facebook

He is correct that many Democrats are angry about what has unfolded in the Oval Office, Congress and Republican-dominated states such as Alabama in the past two years. But he should be careful not to downplay that anger, which drove millions of voters to turn out in 2018 and give Democrats control of the House.

A clear-eyed understanding of the state of American politics is the best way to go in a presidential campaign. If Biden doesn't acknowledge the roots of Trumpism, then it is fair to question whether he can win the election and what he will be able to do with presidential power. But if Biden can engage the critique that progressives have made about his campaign and offer them a realistic path forward, he could turn into a formidable candidate.

Progressives have every right to raise questions when Biden, who was part of an administration stonewalled by the Tea Party, and who downplays the profound ways that polarization has impacted the Republican Party. No longer the party of Lincoln, the GOP is in a very different place than when President Richard Nixon was reelected to office in 1972 -- the year that Delaware first elected Biden to the Senate.