America's new majority: More than half of U.S. households will be Prime members in 2019

Amazon Prime memberships will include more than half of American households this year.But the retail giant wants more, so it's expanding its annual Prime Day to a 48-hour event this year to boost sales and sign up additional Prime subscribers. Despite the milestone, Prime subscriber growth is slowing for Amazon, down to 9% this year from 12% last year.  More than half of U.S. households will have an Amazon Prime member this year, according to data released Wednesday from research firm eMarketer. That makes the service one of the few groups reaching a majority of Americans and underscores Amazon's importance as consumers shift their spending online.  But that's not enough for the Seattle, Washington-based retailer, which is expanding Prime Day into a full 48-hour event this year, running July 15 and 16. The annual shopping holiday exclusively for Prime subscribers dangles deals in a bid to boost sales over the summer — but also to convince more shoppers to sign up for Prime membership.  More on Amazon Prime Day:Amazon warehouse workers plan Prime Day protest over work conditionsAmazon's retail competitors primed to compete on Prime DayAmazon Prime Day 2019: Now you can bet on a site outageDon't make the 4 mistakes on Amazon Prime DayThe best Amazon Prime Day deals right now and the best coming Monday   The growing reach of Amazon Prime has transformed the service into one of the biggest commercial forces in the country, with third-party merchants eager to tap Amazon's huge customer base. Amazon, for its part, is seeking to win customers outside its traditional middle-class base by offering cut-rate Prime memberships to students and low-income Americans on food stamps or Medicaid. "Prime membership is the fulcrum on which Amazon's commerce flywheel spins," Andrew Lipsman, principal analyst at eMarketer, said in a statement. "No event is more important to attracting and retaining subscribers than Prime Day." The research firm projected that Prime Day 2019 could generate more than $5 billion in sales for Amazon, as it continues to drum up consumer expectations around the event. The retailer even streamed a Prime Day concert Taylor Swift headlined Wednesday to woo subscribers ahead of the holiday next week.  The shopping event is especially important for Amazon now as Prime subscriber growth slows. According to eMarketer, that growth rate will slow to single digits this year, down to 9% from 12% last year and 20% in 2017.  "By getting customers to financially and psychologically commit to Amazon, it drives purchase frequency, buying across more categories, and ultimately further entrenches them in the Amazon ecosystem — all right before the all-important back-to-school and holiday shopping seasons," Lipsman added. Amazon Prime by the numbers:  51%: More than half of American households are expected to be Prime members this year, up from 47% last year, projects eMarketer. The number of households tuned into Prime will grow 8.6% this year to 65 mil

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