Speaking generally, endorsements in political campaigns don't count for a lot; they matter more in what you might call the odd case, than in the general run. A major official of the other party endorses you, yeah, that's a substantial deal.
And in a primary, if a very large and crucial organization that works with your party take sides before the nomination, that can be important. And the Oregon AFL-CIO definitely qualifies as a big deal in Oregon Democratic primaries, which is why its endorsement today of House Speaker Jeff Merkley, in his primary contest with Steve Novick for the U.S. Senate nomination, carries some real weight.
The Oregon AFL-CIO has 145,000 members, and it's an active organization. In 2004, the Democratic primary in the state drew (depending on the race) between about 300,000 and 380,000 votes; so the union's votes together with its ground army could matter a great deal. So will the money it raises. The group also noted, "Our endorsement process is stringent and democratic: Candidates must earn the vote of more than two-thirds of our member representatives. Our members’ votes in favor of Jeff Merkley’s endorsement exceeded this threshold."
They also pointed out that they did a comparable endorsement two years ago in the heated primary Governor Ted Kulongoski was caught in; the governor wound up winning it decisively.
Novick is running an energetic and issues-strong campaign, and his press release today on the Superfund and polluter obligations (or what should be obligations) is a good read. But he's running into danger of becoming outgunned.