By Liis Kängsepp 

Estonia's ruling center-right Reform Party and the main opposition Center Party are in a close race in opinion polls ahead of a general election on Sunday that has seen campaigns dominated by socio-economic issues and an increased focus on national security in response to Russia's moves in Ukraine.

A poll published by independent research company Turu-uuringute AS this week, showed that the pro-Russian Center Party is holding a 5 percentage point lead over Prime Minister Taavi Roivas' Reform Party. Another poll published by marketing research company TNS Emor a week ago had the Reform Party as the front-runner with a 1 percentage point advantage over the Center Party.

The Reform Party has been in government since 1999 and seen its support slip by around 5-6 percentage points since the last election in 2011, largely as a result of losing votes to two new populist parties within reach of securing enough backing for a place in parliament for the first time.

The Reform Party is currently leading a coalition with the Social Democrats, which together hold 52 seats in the 101-member parliament.

Election campaigns in this nation of 1.3 million people have touted the need for increased national security amid widespread concerns that Moscow will attempt to destabilize the Baltic countries, including Latvia and Lithuania, in the East-West split that has accompanied the Ukraine crisis. Last year, the alleged kidnapping of an Estonian security agent by Russian officers heightened tensions between the countries, which share a 200-mile border.

"All the parties are in consensus: security is important, we have to invest more into both our defense force and societal security so that we won't have any serious problems with the local Russian minority and scenarios similar to what we saw in Ukraine with the Donetsk People's Republic," Tonis Saarts, a political scientist from Tallinn University, said.

Estonia, like its Baltic neighbors, regained independence from the Soviet Union in 1991 and still has a significant ethnic Russian minority. This minority tends to supports the Center Party, which has a cooperation treaty with Russian President Vladimir Putin's United Russia, while ethnic Estonians votes for other parties.

The Center Party is vying in polls for the top spot but will struggle to form a coalition should it win the most votes. The head of the party, Tallinn Mayor Edgar Savisaar, has riled his political opponents who accuse him of using municipal money for election campaigns, including a television ad featuring Mr. Savisaar as a Superman skater wishing the people of Tallinn a merry Christmas. Mr. Savisaar also made headlines last year when he told public radio program Vikerraadio that the Crimea referendum, which paved the way for Russia's annexation, was "legitimate enough to believe in it."

The three bigger parties--the Reform Party, the Social Democrats and the conservative-right Pro Patria and Res Publica Union (IRL)--have said they won't participate in a government led by Mr. Savisaar.

Mr. Savisaar didn't immediately return requests for comments.

Other issues dominating the election run-up are taxes and wages, including proposals to introduce a progressive income tax and raise the minimum wage. Many Estonians are still feeling the legacy of low wages from the Soviet days as well as cutbacks suffered during the recession that followed the 2008-2009 financial crisis. Many workers, especially outside the capital, are living on the edge of poverty, and want to see higher income, Mr. Saarts said.

Opinion polls also show that two newcomers--the anti-immigration Conservative People's Party of Estonia (EKRE) and the Estonian Free Party--could gather the necessary 5% of votes to nab seats. The parties have campaigned with populist slogans such as "giving the power back to the people."

Data showed that one in every fifth person who had the right to vote cast their ballot online, setting a record for participation at e-elections. Estonia was the first country to introduce nationwide legally binding voting via the Internet in 2005.

Write to Liis Kängsepp at liis.kängsepp@wsj.com