Latvia holds parliamentary elections on October 6 in a vote that could propel a populist, Russian-friendly coalition to power.
Latvia, a member of the European Union and NATO, shares a 270-kilometer border with Russia.
It has a sizable ethnic-Russian minority of around 25 percent, a legacy of nearly 50 years of Soviet occupation that ended in 1991, when the nation regained its independence.
Latvia's mainstream parties have kept pro-Russian politicians from power as they sought ever closer ties with the West.
At the last election, Harmony, a party whose main support is from Latvia's ethnic Russian minority, won the most seats but was excluded from power when the other parties refused to include it in any deal due to its ties with Russia.
But the populist KPV LV party and its leader Artuss Kaimins, whose popularity has soared as he railed against corrupt politicians, has sent mixed messages on whether he would also rule out such a deal.
"We are at a crossroads. Either we will form a new government, or they will," said Prime Minister Maris Kucinskis of the Union of Greens and Farmers party.
A KPV-Harmony ruling alliance would represent "a rather radical change of Latvia's position towards the European Union and towards our security matters which, I think, is very dangerous," he said.