A Macau casino, owned by the eighth richest man in the US and the single largest donor in the 2012 elections – is suing two mainland Chinese high-rollers over alleged gambling debts worth millions.

Sheldon Adelson's Venetian Macau took legal action in a Hong Kong court this week to recover approximately $4.5m (£2.8m). In one lawsuit, filed on Wednesday Hong Kong's high court, the Venetian is trying to recover $3m from Zou Yunyu, who has an estimated $220m fortune from her company, the Shanghai Gaoyuan Property Group. In the second case, the Venetian is pursuing $1.5m from Xie Xiaoqing, a businessman and deputy to the provincial legislature in Hubei province, according to Hong Kong media. The Venetian is one of four casinos owned by the Chinese arm of Adelson's Las Vegas Sands Corp. Adelson's fortune is valued at more than $20bn and his interests span gaming empires in Nevada and Asia, the Israeli newspaper market and Republican politics in the US. In the 2012 elections, he and his wife, Miriam, donated $95m to Republican candidates, including a massive late surge of $33m to pro-Romney political committees.

Macau, a special administrative region of China and former Portuguese colony, earned $38bn in casino revenue in 2012, 13.5% higher than the year before.

Macau's rise to the top of the casino market has been powered by wealthy mainland Chinese gamblers, but the lawsuits highlight the difficulties in collecting debts if they return to the mainland, where gambling is illegal and debts aren't recognised by courts. Both Macau and nearby Hong Kong have separate legal systems from mainland China.

Still, the debts are a drop in the bucket for Las Vegas Sands, which reported net profit of about $350m in the third quarter, with more than 90% of that coming from Sands China, its Macau unit.