No reasonable person, regardless of political belief, would oppose the plan.
Literal
Rational person [conditional-であれば], political beliefs [subject-が] whatever [concessive-であれ], that plan [to-に] oppose [nominalizer-こと] [topic-は] not-would [conjecture-だろう].
であれ (even if it is) is a formal concessive — 'whatever one's political beliefs may be.' 反対することはないだろう (would probably not oppose) is a confident negative prediction. Dense, formal reasoning.