Julian Zelizer, a history and public affairs professor at Princeton University and a CNN political analyst, is the author of " The Fierce Urgency of Now: Lyndon Johnson, Congress, and the Battle for the Great Society ." He's co-host of the "Politics & Polls" podcast. The opinions expressed in this commentary are his own

(CNN) President Donald Trump is undermining his party's ability to succeed. The President, who has squandered five months of united government, has single-handedly made the entire legislative process more difficult for Republicans.

While Trump maintains strong support from his base, congressional Republicans are struggling to work with him on substantive legislative change -- most recently, the repeal and replace of Obamacare.

As Republicans decide whether to take a highly controversial vote that could cost many of them their political careers -- and their ability to influence public debate -- they have not been able to count on the President to remain consistent on message. After House Republicans passed a second version of the health care bill, Trump rewarded them by calling the bill " mean ."

When Senate Republicans struggled to figure out how to salvage a disastrous week on the issue, the President announced that he was " OK " if the vote didn't happen this week.

During the important weeks when Republicans are trying to build support for a major change in health policy, he keeps distracting Americans with one outrageous statement after the other. That's hardly the kind of inspiring oratory that will motivate nervous legislators.

More broadly, there has been no effort to build a message in the last few months on what should be the party's signature legislation. Contrast President Barack Obama, who delivered numerous speeches on his health care bill, including an address to Congress and a televised exchange with Republicans.

Rather than taking the time to build a case with the public like Obama did with the Affordable Care Act or Lyndon B. Johnson did with the Voting Rights Act of 1965 or Ronald Reagan did with tax reform in 1981, Trump has been consumed with tweeting about what he brands as fake news and reiterating that he really did defeat Clinton.

Indeed, right when Republicans need Trump to help put a new health care bill together, he sent an explosive and outrageous tweet about Mika Brzezinski, which caused an uproar Thursday.

There is also evidence he has not paid close attention to the details or the vote. To be fair, this is the kind of information we won't really know for decades, until the presidential archives are opened and we see the records. But several Republicans have commented, anonymously, that he doesn't seem to know what's in the bill or what's going on in Congress with the vote.

Sen. Susan Collins, R-Maine, said openly that his lack of experience has been a problem . In public, many were shocked that he didn't seem to grasp that the Republican bill entails a massive tax cut for wealthier Americans. Perhaps if the President paid as much attention to whipping the vote as he does to watching cable news, Republicans would be in a better place right now. Instead of venting about Brzezinski, his time would be better spent winning over GOP Sens. Dean Heller of Nevada and Ted Cruz of Texas.

But all this might be unfair to Trump. It could be that the substance of this legislation is so problematic and so unpopular that even the most engaged and charismatic president could never have saved this bill. Maybe, just maybe, enough Americans don't believe it's acceptable to strip away health care coverage from millions of Americans whose only crime is to face illness.

According to the latest NPR/PBS Newhouse/Marist poll , only 17% of Americans approve of the Senate bill. And 55% of those surveyed said they outright disapprove of it.

The proposal might not sit well even with Senate Majority Leader Mitch McConnell. In his own home state of Kentucky, The New York Times reports that one out of three residents now depends on Medicaid coverage under the ACA expansion, and in the time that Obama's program has been in effect, the number of people in Kentucky without health insurance plummeted from 18.8% in 2013 to 6.8%.

Regardless, Trump has not helped Republicans on the Hill make their case.

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If health care passes the Senate and then survives the conference committee, it will be in spite of the President's leadership skills.

Republicans like Nebraskan Ben Sasse are desperately trying to salvage the situation. He admitted that Trump's latest tweet was "not normal," and then proposed repealing the ACA -- with a one year delay in implementation. Trump seemed to like the idea, tweeting "if Republican senators are unable to pass what they are working on now, they should immediately repeal, and then replace at a later date!"

Perhaps Sasse's idea could work, though it's unlikely Republicans would support such a decision, given that more people would be harmed by a complete repeal without replace than the current health care bill in the Senate. According to a study by Brookings , Sasse's approach would mean that 30 million more people would lose health insurance by 2019 -- 82% of whom would be working families.

If the bill fails, a large part of the burden will rest squarely on his shoulders. This is something the elephants in Congress certainly won't forget.

This piece has been updated to reflect Sen. Ben Sasse and President Trump's latest comments on health care.