A tug of war is developing over Portugal’s fourth-largest lender, Banco BPI, between its two largest shareholders.

In February, Barcelona-based CaixaBank tabled a €1.1 billion ($1.2 billion) cash offer for the 55.9% of BPI shares it does not already own. But because BPI bylaws limit any single shareholder’s voting rights to 20%, CaixaBank needs to win over 75% approval for its bid to succeed.

That is clearly not the intention of Isabel dos Santos, daughter of the Angolan president and arguably the wealthiest woman in Africa, who controls 18.6% of BPI and has sought to derail CaixaBank’s bid by proposing a merger of Banco BPI with Portugal’s second-largest bank, Millennium BCP. Such a merger would create a dominant presence in Portuguese banking and could run into regulatory hurdles. Moreover, because Angola’s state-owned oil company, Sonangol, already owns 18.4% of BCP, Angolans would become the largest investors in the merged entity.

In her joint letter sent to BCP and BPI, dos Santos argued that the merger would strengthen their presence both in Portugal and in Portuguese-speaking Africa, primarily Angola and Mozambique. In contrast to this Lusitanian banking empire, CaixaBank is seen by some as the “Spanish raider.”

In fact CaixaBank’s roots are Catalan, but it has expanded through mergers and acquisitions to become Spain’s largest banking network. Should the BPI takeover succeed, it would make CaixaBank the market leader across the Iberian peninsula.

Separately, BPI is one of 15 potential bidders approved by the central bank in the auction of Novo Banco, the “good bank” salvaged after the collapse of Banco Espírito Santo last summer. A high enough bid to win this prize might also act as a poison pill—or deterrent to a hostile takeover.

Which, since BPI’s board has rejected CaixaBank’s initial offer, is where things stand. BPI shares bounced following dos Santos’ intervention and the directors’ rejecting the bid but have since trended downwards. A report from the Portuguese investment bank BESI suggests that CaixaBank cannot raise its offer by much without negative impacts to its own capital ratios.