The nation’s first offshore wind farm needs buyers for half its power, and one of its best prospective customers has been reluctant to sign on. But a pending merger involving the utility NStar has become a possible pressure point to get the company to buy Cape Wind power. Since NStar’s merger with Northeast Utilities was announced, Massachusetts officials have moved to ensure that regulators don’t approve such mergers unless they advance the state’s clean energy goals. Those goals include developing offshore wind. The state has also requested a stay in the merger proceedings that could lead to a merger-killing delay. Cape Wind critics say the state is trying to force NStar to buy a favored project’s power. But the state says it’s just upholding state law and protecting the public interest.