The union alliance endorsed Kennedy on Monday, the Boston Herald reported. The Service Employees International Union State Council backed him last week.
Kennedy, a member of the country's best-known political dynasty, is considering his first run for office. The 4th Congressional district has been redrawn and is less heavily Democratic than the one now represented by Rep. Barney Frank, D-Mass., who is retiring.
Sean Bielat, who is seeking the Republican nomination and ran against Frank two years ago, was not surprised by Kennedy's endorsements.
"Well, that is shocking," he told the Herald. "It's the first step in the coronation process. This is how the Kennedys do things."
Kyle Sullivan, a spokesman, said Kennedy does not take the seat for granted.
"He has been taking some time to get across the district to listen to people's thoughts on the issues facing families of the 4th District as well as their thoughts on his potential candidacy," Sullivan said. "If he runs, he is going to work hard to earn every vote."