From the article:
So what’s the difference between those earlier surveys and today’s NBC poll? Republicans. In every previous survey, Republicans were most likely to support attacking Syria. Each poll showed more than 50 percent of Republicans willing to strike Assad if he used chemical weapons. Today’s NBC News poll shows far less Republican support, with just 41 percent in support and 49 percent opposed. That’s 15 points less than April’s Pew Research survey, which found that 56 percent of Republicans would support strikes. In comparison, Democratic support hasn’t declined—46 percent support strikes, just like in April.