Willis Group Holdings and Towers Watson, facing some opposition, are working aggressively to shape stockholder opinions in favor of their proposed merger in advance of a Nov. 18 shareholder vote scheduled at both companies.

They do so as strong opposition continues from Driehaus Capital Management, a Chicago-based asset manager that owns more than 1.1 million Towers Watson shares.

Driehaus disclosed its opposition earlier this fall, conditioning it on improving the deal terms. Driehaus issued a statement on Nov. 2 celebrating strong earnings at Towers Watson and a continued argument against the deal.

“The report speaks to the impressive standalone prospects for Towers Watson and offers further evidence that the Willis proposal substantially undervalues TW shares,” Drihaus said. “We are confident that the Willis proposal will be rejected unless terms are materially improved. We once again urge fellow shareholders to vote against this value-destructive deal.”

In the wake of this, Willis and Towers Watson issued extensive public statements that attempted to poke holes in merger assessments issued by Institutional Shareholder Services and Glass, Lewis, & Co. Both Willis Group and Towers Watson took particular issue with the ISS assessment, which concluded that Towers Watson shareholders should not support the merger.

“Naturally we are pleased that ISS recognizes that the ‘strategic merits and long-term benefits of the merger,’ and with their recommendation that Willis shareholders should vote for the transaction,” Willis CEO Dominic Casserley, said in prepared remarks. “However, we are disappointed with their conclusion that Towers Watson shareholders should not support the merger.”

Casserley explained that Willis, a global reinsurance and insurance broker, believes “that this perspective neglects the estimated $4.7 billion in incremental value for shareholders that we expect, through clearly identified cost, tax and revenue synergies.”

Towers Watson, a professional services and analytics firm, said in its prepared markets that “we respectfully disagree with the conclusion reached by ISS and Glass Lewis.”

Towers Watson explained that “while clearly acknowledging the sound strategic rationale and synergy potential of the transaction, they focus on short-term trading, take a narrow view of relative value contribution and unduly discount the significant long-term value creation potential of the proposed merger with Willis.”

The proposed Willis/Towers Watson all-stock merger of equals, announced in June, was initially valued at $18 billion. Once complete, Willis shareholders will own about 50.1 percent and Towers Watson shareholders would own 49.9 percent of the combined company, which will be named Willis Towers Watson. It would be domiciled in Ireland, where Willis Group moved its place of incorporation in 2010.

The companies said they’ll produce $125 million in annual merger related cost savings and about $75 million in annual tax savings.

Sources: Towers Watson, Willis, Driehaus Capital Management