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) Republicans allowed a crucial farm bill that renewed major agriculture programs to fall apart over immigration on Friday, in the most recent demonstration of why the party should be worried about losing control of Congress in the 2018 midterm elections. Each special election brings more evidence that Democrats are motivated, enthused, and prepared.
The large number of successful Democratic female candidates suggests that the women's vote might be unusually large. The turbulence in the Oval Office has taken its toll on Republicans. This presidency has given Democrats the best opportunity since 2008 to recapture Congress.
But in American politics, opportunity does not automatically translate into success. Opportunities can be botched. And while the odds are very strong that Democrats will eat into the size of the Republican majority -- after all, most midterm elections involve the party of the president losing seats -- there are ways in which the GOP can contain the damage from the "blue wave."
Republicans have a number of serious political assets that make them a formidable foe to the Democrats.
In the crucial battle for voter turnout, the GOP will count on three constituencies who have been very pleased with President Donald Trump's absolute loyalty to their agenda: gun rights advocates, evangelical Christians, and anti-immigration hardliners. Despite all of the craziness from this administration, the President has been very consistent and predictable in giving his support to these core groups.
The reason that their loyalty matters is that all three groups tend to be well organized, well financed and to command sizable numbers. As the reality of the midterms approaches and Republicans amplify the threat of a Democratic Congress that will move forward on impeachment, these groups of voters will likely accelerate their efforts and drive to get out the vote.
And if fatigue sets in among Democratic opponents of President Trump, they might not be able to deliver on their turnout. There have been so many scandals and provocative moments since January 2017. Democrats could become so tuned out -- tired of being angry or watching more arguments -- by the time we reach November that their enthusiasm may not be as high as expected.
Democrats will also have to contend with the perceived successes of the GOP. Barring any unexpected downturn, Republicans can benefit from the economy. If current trends continue, by most standard measures the economy could be doing well, come November. Unemployment is low, stock markets are booming and consumers are feeling more confident. Some voters might decide that the status quo is worth it, irrespective of all the eye-rolling presidential tweets.
Although most midterms revolve around domestic issues, foreign policy is something that could become part of the election mix. Despite the current fallout from Trump leaving the Iran nuclear deal and the tensions in Gaza, Republicans are anticipating the potential of a major diplomatic breakthrough with North Korea. Each of those could have the capacity to sway the electoral playing field, much as the Cuban Missile Crisis in October 1962 helped Democrats maintain strong majorities in the House and Senate.
Finally, Republicans still have some built-in advantages that make the election an uphill climb for the Democrats. In the Senate, Democrats still have to knock off a significant number of GOP-held seats while defending a few vulnerable ones. Gerrymandered safe seats also offer the GOP some insulation in the House by limiting the number of seats that are really in play. Republicans also have significant fundraising powerhouses to count on. Massively wealthy donors like the Koch Brothers and Sheldon Adelson are pouring money into campaigns to make sure that the deregulatory direction of policy under Trump is not brought to an end.
None of this means that Republicans are in great shape. They are not. Democrats have history on their side and they will benefit from the unpopularity of Trump.
Yet the size of Democratic gains will matter, because it can determine the difference between continued unified government or a divided Washington. As difficult as it is to believe, given the controversial presidency of Donald Trump, Republicans still have more than enough reason to believe that they stand a chance to ride rather than drown in the blue wave.