By Elida Moreno and Sean Mattson
PANAMA CITY (Reuters) - Panamanian lawmakers on Saturday passed a bill that would require a run-off in close presidential elections, but opposition to the proposal is deepening the split in the country's ruling coalition.
The bill, which faces another vote for approval, calls for a second round if no candidate wins more than 50 percent in the first round.
Currently, a candidate in a race with three or more participants can win by garnering the most votes.
President Ricardo Martinelli heralded the reform as a democratic advance for the fast-growing Central American country. But critics say the bill, approved 42-12, is a strategy to keep his Democratic Change Party in power.
The debate occurred as political wrangling breaks up the coalition that put Martinelli in office. Martinelli, a supermarket mogul, has denied he will seek re-election in 2014.
Martinelli fired Juan Carlos Varela as his foreign minister this week after breaking a deal to back Varela for president in 2014. Varela's Panamenista Party helped elect Martinelli in 2009 and Varela still holds the post of vice president.
Finance Minister Alberto Vallarino then resigned in support of Varela, but he will stay on until a replacement is named.
Some Panamenista lawmakers have broken ranks to vote with Martinelli's side in the run-off debate. But others are voting against the president, whose party holds 33 of 71 seats in Congress.
The turmoil may be undermining popular support for Martinelli. A poll published in La Prensa newspaper showed his approval rating fell to 52 percent in September from 64 percent in June and 86 percent in October 2009.
Martinelli has impressed Wall Street credit rating agencies with reforms that have boosted growth but kept debt in check. Analysts at Barclays Capital said in a note that the political discord could end up hurting the country's institutions.
The run-off bill needs another vote in the unicameral legislature, but its initial approval all but guarantees passage. Opposition lawmakers said they will challenge the law in court.
A lawsuit will test the independence of the courts from political influence, said Miguel Antonio Bernal, a law professor at the University of Panama.
"The law is clearly unconstitutional," he said.
(Writing by Michael O'Boyle in Mexico City; Editing by Xavier Briand)