Chris Stirewalt of Fox News probes new polling data that gauge the Affordable Care Act’s impact on likely voters in the 2014 campaign.

How big is ObamaCare in this year’s midterms? More than 80 percent of respondents in a new poll conducted by the Pew Research Center for USA Today said that a candidate’s stance on the troubled health law is important to them, with 54 percent saying it is “very important.” The worst news for Democrats is that among the majority of registered voters who said the law was “very important,” there were twice as many opponents as there were supporters. Overall, support for the signature Obama initiative, which pairs expanded welfare and entitlement programs with far-reaching regulations on health insurance, fell to 37 percent. That’s the lowest level since the summer after the law was passed in 2010 when incumbents’ town halls across the country erupted with ObamaCare outrage. The takeaway: The electoral climate this year is defined by ObamaCare and the forecast is getting worse for Democrats. …

… Some race watchers may deduce from the poll that the constant efforts by President Obama and his fellow Democrats to frighten and mobilize base voters will be inadequate in the face of an overall political climate as dire as the one described by the Pew/USA Today poll. But the survey actually shows the importance of efforts to provoke a stampede of liberals to the polls. With centrist voters trending away from Democrats and a majority of independents beyond the reach of Democratic pleas for more mercy on the law, an agitated Democratic base could make the difference between just losing the Senate majority and a rout. If the GOP gets a major wave going, it could Republicans in a place to actually pass legislation rather than just blocking Obama’s efforts.