By Emre Peker and Yeliz Candemir 

ISTANBUL--Turkey's currency slumped to its lowest since Prime Minister Recep Tayyip Erdogan led his party to a decisive victory in March local elections, with a broader emerging-market rout hitting the lira hard just four days before the country's first popular presidential vote.

The lira slumped by as much as 0.84% to 2.171 per dollar by midday on Wednesday, the lowest since March 31 and the leading loser among emerging-market currencies. The lira has fallen by 3.5% since July 25, when Turkish markets went on a five-day post-Ramadan holiday. The government's benchmark two-year bonds also fell, with yields soaring to a three-and-a-half-month high of 9.67% from 8.74% last week.

As global markets drop on tensions driven by the Ukraine conflict, domestic concerns led by central bank's dovish monetary policies, persistent government calls for interest rate cuts amid stubbornly high inflation, and speculation that Moody's Investors Service could downgrade Turkey to junk status are once again stoking volatility in Turkish assets.

"We expect the negative sentiment to continue," said Nilufer Sezgin, chief economist at Erste Securities in Istanbul.

The market rout comes as Mr. Erdogan is poised to clinch the presidency on Sunday, potentially extending his rule for another decade in a bid to more than double Turkey's $820 billion economy. The premier is polling an average 54% support, compared with under 40% for the joint-opposition candidate.

But challenges abound, especially with U.S. data signaling an economic boom that could herald the beginning of interest rate rises by the Federal Reserve. That would start draining the flood of cheap cash that had been underwriting growth and plugging funding shortfalls in emerging markets such as Turkey, reversing the flow of money back to developed economies.

"As the U.S. economy appears to be stirring from its pre-summer siesta, an air of unease is beginning to pervade emerging markets' shores once again," said UBS strategists led by Bhanu Baweja in London. "The market has moved forward and priced in more aggressive action from the Fed next year."

The anticipated policy tightening in the U.S. and in other emerging markets jar with calls for a looser stance in Turkey, where government officials persistently press the central bank to support economic growth by reducing borrowing costs.

This week, Economy Minister Nihat Zeybekci once again called on Governor Erdem Basci to cut interest rates, even as data on Monday showed inflation accelerated to 9.32% in July from 9.16% in June.

The jump defied central bank forecasts that inflation would start slowing down in June, rising instead to almost double the 5% official target. Resurgent inflation also indicates that contrary to Mr. Basci's claims, price increases are broad-based and not restricted to food costs.

"Contrary to [market] calls that interest rates shouldn't be lowered at the next Monetary Policy Committee meeting because inflation is higher than expected, we [the government] argue that rates should be cut until the expectations of producers, investors and exporters have been met," Mr. Zeybekci said on Tuesday. The central bank's next decision is due on Aug. 27.

Turkey's economy minister also said his expectations are on the "negative side" with regards to the Moody's report on Turkey due on Friday. His comments were echoed by Salih Kapusuz, a lawmaker from Mr. Erdogan's governing party.

"We know that credit-rating firms like Standard & Poor's, Moody's have previously cut Turkey's credit rating on subjective criteria," Mr. Kapusuz said Tuesday in messages from his Twitter Inc. account, referring to a belief that the country's credit ratings are driven by political calculations, and not economic merits. "There is speculation of certain efforts to cut Turkey's credit rating before elections, we hope they're not true," he said."

Economy Minister Zeybekci later said in a tweet that he doesn't anticipate a downgrade from Moody's. On Wednesday, Deputy Prime Minister Ali Babacan, who is in charge of all the economy-related ministries, also said the Treasury hadn't received a signal from Moody's about a rating cut.

A London-based spokeswoman for the ratings firm pointed to Moody's ratings release calendar and declined to comment on how Friday's report may affect Turkey's Baa3 rating, the lowest investment grade, which carries a negative outlook.

Turkey's central bank has been cutting its benchmark rate since May, when the current-account deficit declined to $48 billion from $65 billion in 2013--shrinking to 5.8% of gross domestic product from 8%--and as exports helped rebalance economic growth away from credit-fueled consumer spending. Policy makers have reduced the one-week repo rate to 8.25% from 10%, and they are expected to continue easing their stance.

"In late August, the central bank is likely to, for the fourth time in a row, yield to Erdogan's pressure to cut interest rates to stimulate growth," said Amy Yuan Zhuang, an analyst at Nordean Bank AB. "Turkey's large external financing need combined with the prospect of less easy monetary policy from the Fed will curb capital inflows, also lira negative."

Write to Emre Peker at emre.peker@wsj.com and Yeliz Candemir at yeliz.candemir@wsj.com

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