If the court strikes down Obamacare in June of 2020 then 5 months before the election the pre-existing coverage requirement, the elimination of caps on coverage requirement, and the coverage of dependents up to age 26 requirement all are struck down as well. While Obamacare as a whole may not be overly popular, those three aspects of it are the exception. So five months before the election the Republicans will have to explain why they eliminated those three popular features and the Democrats will have an issue that they can run against. And if McConnell really thinks that Congress would "act quickly, on a bipartisan basis" to fix it then he really is a colossal idiot.
Republicans didn’t eliminate it if it was unconstitutional. In fact they’ve done next to nothing to interfere with it.