The ruling party's leaders are racking their brains over the political reform bill.

Literal

Ruling-party [possessive-の] leaders [topic-は] political-reform-bill [over-で] wisdom [object-を] are-squeezing.

知恵をしぼる ('rack one's brains / think hard,' literally 'squeeze wisdom') is a set idiom for intense thinking. しぼる is the verb 'squeeze' (as in squeezing juice). A common idiom for mental effort.