Julian Zelizer , a history and public affairs professor at Princeton University and a New America fellow, 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) Gamblers are likely betting on Democrats to do quite well in the 2018 midterm elections.

Historically, the party of the president gets a shellacking, as President Obama called it in 2010, in the first midterm that they face. There have been only two cases when the president's party gained in both houses of Congress since the Civil War: in 1934 and in 2002.

Besides the normal historical cycle of backlash against the White House, Democrats this time around are whetting their lips because House Republicans have just voted for a health care bill that rescinds benefits for millions of hard-working Americans.

This could prove to be a policy victory that ends in political defeat. Between the midterms' track record and the polarizing health care legislation, this week's events would seem to be a recipe for political disaster for the GOP.

Democrats, on the other hand, see a silver lining in the House vote. They are hoping that TrumpCare will become the Republican ObamaCare, producing the same flip of control of the House that Democrats suffered in 2010.

According to David Wasserman of the Cook Report, one of the shrewdest political observers around, the vote "guarantees Democrats will have at least one major on-the-record vote to exploit in the next elections."

And according to one Republican political strategist who deeply dislikes the president, "What we've done here is political malpractice. Democrats will run ads with weeping parents who can't cover their premiums and Little Johnny dying ..."

A sweep for Democrats?

But is a Democratic sweep in 2018 really so certain? Is it so obvious that Republicans are on the verge of total electoral disaster?

The answers are not as clear as they might seem. Despite the House vote, President Trump and the GOP are looking at some pretty good economic numbers today.

With the economy having reached full employment, the best conditions in more than 10 years, many voters will be in good spirits about the status quo. Notwithstanding all the talk about the impact of the health care legislation, the bottom line to Americans' pocketbooks will matter a great deal come the midterm campaigns.

If conditions don't change significantly, Republicans will benefit. President Trump and the GOP, whether they deserve it or not, will be able to claim credit for the recovery. (Presidents usually get the blame or credit for economic conditions, even if they don't have a big impact on them.)

Republicans will say that indeed he is making America great again. Trump's supporters will feel that he delivered. Many Republicans who never loved Trump will nevertheless be pleased with the state of economic affairs.

There has been much discussion about the current version of the House bill and its "losers" who will make their angry voices known. But there are winners as well who will support what the GOP has done: wealthy Americans whose taxes will be lower, upper-middle-class Americans without pre-existing conditions, younger middle-class people who don't want insurance or want more meager insurance, and large employers who don't face the the mandate to offer coverage as they did under Obamacare.

To be sure, Republican candidates will face the ire of hospitals, Medicaid recipients and elderly Americans who are furious about the impact of the reform, if it passes. But this is not a bill that only takes things away from voters. Indeed, the bill redistributes benefits to some very powerful sectors of the country.

Benefits of winning

The benefits of winning in politics are important. If a health care measure gets through the Senate, Republican voters will be energized about a watershed victory. On the campaign trail, Republicans will have something to brag about when discussing what they have achieved in the Trump Era.

It is also very possible that passing health care would open to doors to a major supply-side tax cut that would bolster the confidence of core Republican constituencies who have money to spend on campaigns.

Democrats who are counting on a backlash to Trumpcare should also be careful not to forget that we live in a political world where public perceptions of policy and government are not simply the product of rational calculations by voters.

They learned this in the 2016 election but seem to have forgot the lesson. Partisan forces, ranging from the president to biased media outlets, can have a profound impact on how voters interpret what is happening in the political world.

Master of spin

Republicans have proven to be enormously successful at shaping the narrative about government decisions and policy. President Trump is a master of spin.

As the impact of a health care bill becomes clear, statistics won't be the only thing that matters. Democrats will have to counteract the powerful public relations campaign that President Trump and Republicans will certainly mount to convince voters that all the positives in health care are a product of their efforts and all the problems are still legacies of President Obama's broken policies.

The midterms are more than 18 months away, and a lot can happen to shape how they will play out.

Yet Republicans can still count on gerrymandered districts to protect incumbents in a large part of the red map. While courts have forced some states to redraw districts put into place after 2010, many of them remain in place for 2018.

With the largest GOP majority in the House since 1928, it won't be easy for Democrats to retake control of Congress. Democrats will have some heavy lifting to do, even with history on their side.

Math favors Republicans

In the Senate, as CNN's Chris Cilizza recently wrote, Democrats must defend 25 seats. The Republicans are defending only eight. Ten of the states Democrats need to protect are in states that Donald Trump won. In North Dakota, Missouri, Montana, West Virginia and Indiana, he won by whopping margins.

Nor is it likely that the House bill will be the final legislation. Senate Republicans are already toying with a number of changes, such as boosting tax credits for low-income Americans or delaying the end of the Medicaid expansion that would make the bill more politically palatable in the short-term.

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If they do, and Freedom Caucus Republicans in the House decide that they need to deliver some kind of legislation, even if flawed, this could fundamentally change the dynamics of how the bill is perceived in the near future.

The reality is that Democrats should not be too giddy about the recent vote and their prospects come 2018. It will be a long slog to retake control of Congress and even to put a significant dent in the size of the Republican majorities. And Republicans can make a credible case that they will retain control of Congress and carry out more of their conservative agenda in the years ahead.