By Gram Slattery

VALPARAISO/SANTIAGO (Reuters) - Chilean voters are expected to shift right in local elections this weekend, dealing a blow to the center-left governing coalition and serving a further setback to Latin America's already struggling leftist movement.

Center-left President Michelle Bachelet was elected three years ago with high approval ratings, and her Nueva Mayoria coalition, though riven internally, has held firm control of Congress.

But as Chileans prepare to vote for mayors and local councils this Sunday, it appears the left's era of dominance is drawing to a close, with likely consequences for next year's presidential election.

A series of high-profile corruption scandals among politicians, in a country that prides itself on its probity, have angered Chileans, and both the government and opposition have seen approval ratings drop into the teens.

That anger will hurt Bachelet's coalition the most, analysts and politicians say, as voters set out to punish incumbents. In the local elections, right-wing parties are expected by pollsters and politicians consulted by Reuters to make gains, especially in important swing districts.

That would likely portend a strong performance in presidential and congressional elections in November 2017, in which Bachelet is constitutionally barred from running.

According to a September poll, 18 percent of Chileans wanted the right-wing coalition's likely candidate, Sebastian Pinera, to become the next president, versus 5 percent for Ricardo Lagos, the front-runner to represent the Nueva Mayoria.

Both are ex-presidents and have not declared their intention to run, though both have put forth policy proposals and hinted heavily at bids.

"If the [presidential] election were held this weekend, I'd have to say the right would win," Socialist Party lawmaker Osvaldo Andrade, president of Congress' lower chamber, told Reuters.

The shift would be further evidence of a retreat from left-leaning ideologies in Latin America, where leftist governments including in Brazil and Argentina have lost power.

But few Chileans are enthusiastic about the right-wing opposition, either, and new parties are threatening a shake-up of the two-coalition system that has dominated politics since Chile's 1990 return to democracy.

The number of official parties has increased to 32 from 15 in the last election cycle, according to Chile's electoral service, most of them left of center.

Though none are expected to make a splash on Sunday, analysts say they will likely make inroads at the 2017 vote, in which new electoral rules will increase their chance of winning power.

"The traditional political system, as we've known it, will cease to exist," said center-left lawmaker Pepe Auth.

"Today's voters aren't going to move from one party to another; they're going from the parties to no affiliation at all."

BATTLE FOR SANTIAGO

The mayoral race for central Santiago is widely seen as a barometer for the national political climate.

The district is home to Chile's presidential palace and Supreme Court, as well as an unusual mixture of professionals and working class residents that make it both symbolically important and attainable for conservatives and leftists.

Left-leaning mayor Carolina Toha, seen as an important figure in the opposition to the 1973 to 1990 Augusto Pinochet dictatorship, won the 2012 election with over 67 percent of the vote.

But internal party polls from both coalitions show this year's race as a dead heat between Toha and conservative Felipe Alessandri.

A victory for Alessandri here, analysts say, would serve as an important symbol for the right's resurgence.

However, both candidates told Reuters the biggest battle will be getting supporters to the polls. Only around 30 percent of Santiago's disaffected voters are expected to vote.

"Rather than beating Toha, I have to beat abstention," said Alessandri. "That's my challenge, to motivate people, to make them return to believing."

(Reporting by Gram Slattery, Editing by Rosalba O'Brien; Editing by Alistair Bell)