Estonia's center-right government is being challenged by a party that draws its support from the Baltic country's Russian minority. Here are five things to know about Estonia's parliamentary vote on Sunday:

UKRAINE FACTOR

Security is a key concern for Estonian voters following Russia's intervention in Ukraine. Even though Estonia is a NATO member, many are worried that Moscow may try to boost its influence in the country, a former Soviet republic where about one-fourth of the 1.3 million residents are ethnic Russians.

Security policy is a rare issue on which the main political parties mostly agree. All favor robust defense spending and a strong presence of NATO forces in Estonia. However, the opposition Center Party — the party of choice for most ethnic Russians — favors a friendlier approach to Moscow to balance ties with the West.

FLAT TAX

Estonia is known for its pro-market policies and small public sector. It was among the first European countries to introduce a flat tax on personal income, currently set at 20 percent. Companies pay no taxes on reinvested earnings, a rule meant to help startups and other firms expand.

Analysts say the flat tax is likely to stay though two parties, the Social Democrats and the Center Party, support introducing progressive taxation instead.

E-VOTING

Estonia is a pioneer in electronic voting, becoming the first country to implement Internet voting in a nationwide election in 2005.

Election officials say more than 176,000 voters chose to mark their ballots electronically this year, an increase of 25 percent from 2011.

The key to the system is Estonia's national ID card — nearly all residents have one — complete with a microchip. Together with a card reader and computer, it allows voters to log on to an election website and cast a vote.

WHAT THE POLLS SAY

Recent polls indicate that the Center Party, led by Tallinn Mayor Edgar Savisaar, will finish first in the election with more than 25 percent of the vote. Still, it will probably have to remain in opposition.

The other major parties, Prime Minister Taavi Roivas' center-right Reform Party, the conservative IRL party and the centrist Social Democrats, refuse to join forces with Savisaar because of his alleged close ties with Moscow.

Polls also show two new parties may exceed the 5 percent threshold to break into the 101-seat Parliament: the nationalist EKRE party and the conservative Eesti Vabaerakond.

Estonia's current two-party coalition of the Reform Party and Social Democrats holds a narrow majority with 52 lawmakers.

NON-CITIZENS

More than 80,000 of Estonia's estimated 330,000 ethnic Russians can't vote in the parliamentary election because they are not Estonian citizens.

Though many of them have never lived anywhere else, they cannot — or don't want to — meet the citizenship requirements introduced after Estonia regained independence in 1991. A major obstacle is mastering Estonian, which is not a Slavic language like Russian.

The situation often draws criticism from human rights groups as well as Moscow, which accuses Estonia of discriminating against its ethnic Russian minority.

Those defending the citizenship rules say national laws define Estonian as the only official language, and that it's not unreasonable to require all citizens to have at least basic knowledge of the language.