A popular way to bet on Turkey's stock market was tumbling sharply on Wednesday, as the Turkish government prevented foreign banks from accessing lira in so-called swaps transactions, a tactic intended to prevent bets against lira ahead of a weekend election, according to reports. That move has sent the overnight cost of borrowing lira, or swap rate, to 1,200% from 23%, according to Reuters. The exchange-traded iShares MSCI Turkey ETF was down 7.8% midday Wednesday as the Turkish lira was weakening sharply against bucks. The greenback last bought 5.4272 lira, up 1.9% against the Turkish currency, according to FactSet data. Volatility in Ankara's market comes ahead of a key vote on Sunday, with incumbent President Tayyip Erdogan fighting to keep his AK Party in power. Turkey's economy has been struggling with high, double-digit consumer-price inflation, an ailing currency and rising interest rates in the U.S., which had made Ankara's dollar-denominated debt burden more expensive in local currency terms. Erdogan has called for a ban on the use of U.S. dollars, which has sapped liquidity in markets that trade lira, including currency-trading hub London.