Once a progressive, Mr. Romney now tries to present himself as an ardent conservative, a policy that has allowed the other candidates to denounce him as a flip-flopper and an impostor.
想问一下为什么这个句子是正确的。最后那个a policy充当的什么成分。主谓宾应该是mr.r tries to present himself....这里吧,那个a policy不会多出来么?