Editor’s Note: Julian Zelizer is a history and public affairs professor at Princeton University, editor of “The Presidency of Barack Obama: A First Historical Assessment” and co-host of the “Politics & Polls” podcast. Follow him on Twitter: @julianzelizer. The opinions expressed in this commentary are his own.

CNN  — 

Democrats have a history of underestimating Republican presidents.

Ronald Reagan, numerous Democrats originally thought, was a lightweight Hollywood actor with charisma and television appeal but not much more. George H.W. Bush, according to his critics, was a well-meaning “wimp” whose leadership skills were lacking and who could never escape from the shadow of Reagan.

His son George W. Bush, Democrats joked, was a nice guy who you might want to have a beer with but someone who didn’t know much about world or domestic affairs. Americans who saw the televised debates in 2000 can probably still hear the sound of Vice President Al Gore sighing after almost every remark.

In each case, however, the Democrats didn’t see what was coming.

Reagan went on to be a two-term president who vastly expanded military spending, slashed corporate and individual income taxes, lowered spending for much of the social safety net and negotiated a historic arms agreement with the Soviet Union.

Though he couldn’t win re-election in 1992, President George H.W. Bush led the nation into its first major military operation since Vietnam with Operation Desert Storm, reached a historic deficit reduction agreement with the Democratic Congress in 1990 and he presided over the collapse of the Soviet Union.

Under President George W. Bush, who left Democrats shell-shocked when he won a second term in 2004, the nation saw the administration vastly expand the national security state after 9/11, launch two major military operations in Afghanistan and Iraq – the second disastrous – cut income taxes even further and withdraw from a major climate change accord, while pushing through Congress several major legislative initiatives such as No Child Left Behind and the Medicare prescription drug plan.

The last two weeks have been another a loud wake-up call for Democrats who have railed against President Donald Trump but who thought that this reality star commander-in-chief was so incompetent, corrupt and self-centered that it was only a matter of time before he went away.

In their view, the President who surrounded himself with third-rate advisers and who had no legislative skills to speak of would be hampered in how much damage that he could inflict before his term ended.

It’s looking very different right now.

The minute that Supreme Court Justice Anthony Kennedy announced he was retiring, every Democrat who has been paying attention to the Court realized the implications. With the evangelical right cheering him on, President Trump now has the opportunity to push through a giddy Republican Senate a judicial nominee who will dramatically swing the majority of the Supreme Court much further to the right.

Trump’s sway

With Justice Kennedy’s announcement coming at the same time that the court announced that it was dealing a major blow to public employee unions and upholding the president’s controversial travel ban, the implications of Trump’s sway over the highest court in the land were immediately apparent.

Whoever is president after 2020 will be dealing with a Supreme Court majority that has much less tolerance for strong intervention by the federal government and will be less supportive of the rights-based policy gains that have vastly strengthened the social standing of African-Americans, gay Americans, women and others who have suffered marginalization for decades.

Policies such as abortion access, family planning, affirmative action and voting rights now hang in the balance.

If Democrats were thinking that President Trump’s blistering rhetoric about undocumented immigrants was just talk, they now know just how far the President is willing to go. He is very serious about closing the borders and the Supreme Court’s decision to uphold his travel ban will embolden him.

Although he was pressured into backing down from his draconian policy of separating children from their parents at the border, Trump sent his message loud and clear. He is willing to go as far as he thinks is necessary to fight for stringent border policies. He is willing to inflict psychological damage on kids and subject border crossers to the toughest security measures possible until he convinces Congress to build the physical wall that he has been promising.

While he backed away from the policy of family separation, he is seeking congressional authority to detain entire families for longer than 20 days.

He has already dismantled President Obama’s Deferred Action for Childhood Arrivals (DACA) program, leaving hundreds of thousands of young people’s lives in America under a cloud. Their future depends on a Congress that is unable to reach agreement on any legislation dealing with immigration. Indeed, the president used his Twitter account to effectively torpedo an effort last week to pass compromise legislation on immigration that would have fixed the crisis he created with the Dreamers.

His sympathy remains with the hardline anti-immigration elements in the Freedom Caucus who will keep pushing for tighter and more restrictive policies on immigration – both undocumented and legal.

Last week’s Gallup polls and the results of Tuesday’s Republican primaries in South Carolina and New York are also strong indications that his political support remains much more substantial than Democrats had expected.

In the months that followed the inauguration, the conventional wisdom had been that as his national approval ratings kept falling, his political support within the GOP would follow. But we can see from the recent polling that Republican support for the President remains rock-solid and seems to be getting stronger.

Despite all of his chaotic and controversial decisions, his national approval ratings in some polls have even crept upward to the range of 45%. With a low rate of unemployment and a booming stock market, there is reason to believe that those numbers might hold fairly steady.

More of a destroyer than a builder

Trump is also demonstrating that the power of the President to tear things down is immense, especially if that President is not particularly interested in putting something different in its place. Interestingly enough, the real estate developer President has not turned out to be much of a builder. He prefers to take things apart and then walk away from the rubble without looking back.

Short of obtaining repeal and replace, he has severely weakened the Affordable Care Act by taking smaller steps like ending the individual mandate. He pulled out of TPP, pulled out of the Paris Climate Accord and pulled out of the Iran nuclear deal. He has issued executive orders rolling back Obama-era regulations to curb climate change and constrain Wall Street.

While Congress and the courts have significant power when it comes to checking legislative initiatives from the Oval Office, a president who is intent on dismantling policies – such as stripping away regulations or withdrawing from international agreements – can get a lot done if he or she is determined. A president who wants to use the bully pulpit to undercut the public confidence in institutions, such as the news media or law enforcement, can do great harm if they don’t care about the long-term consequences.

As the dog days of summer begin, Democrats should be more concerned than ever before about the consequences of a Trump presidency. The possibility for President Trump to seriously transform American policy keeps growing and the potential for a two-term presidency can no longer be dismissed. This unstable, shallow television star is starting to demonstrate that he has some very real political muscle to keep pushing forward.

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    The stakes of the 2018 midterm elections should be clear. If the national party does not figure out how to put forth an effective campaign that generates high turnout and excites the passions of their electorate, and if they don’t engage in the upcoming Supreme Court confirmation fight in a way that slows down the process and uses the President’s pick to awaken voters to the stakes of this struggle, President Trump could be looking at two more years of united government, with a GOP that will see him as an influential kingmaker, and the Congress will be more willing to start handing him legislative victories on the path to 2020.