Starbucks' endorsement of gay marriage has improved its perception among Democrats, but it has turned Republicans sour.

In January, Starbucks was among the more than 100 Washington-based companies to support a bill legalizing gay marriage in the state. Lawmakers approved the measure and Governor Chris Gregoire signed it into law. Currently, the law is on hold pending the outcome of a citizen's initiative to be held in November.

(Related: Washington gay marriage supporters outraise foes in June.)

The National Organization for Marriage (NOM), which is opposed to such unions, launched a boycott against the coffee giant. As of Tuesday, 45,874 people had pledged at the group's boycott site to “dump Starbucks.” However, more than 651,145 people joined a petition thanking Starbucks for its support.

In January, both Democrats and Republicans perceived the brand equally, according to YouGov BrandIndex's Buzz score. Democrats scored Starbucks at 15.7 and Republicans at 14.6. By last week, however, a clear partisan divide had emerged. Perception among Democrats improved to 19.3 and among Republicans declined to 10.7.

“I don't know about you, but I would be pretty unhappy if this was how my 'non-political' coffee company was being viewed by conservatives in the United States,” NOM wrote at its blog.