Sweden's Christian Democrat Party has voted to push for the country's NATO membership, meaning that all four conservative opposition parties now favor joining the alliance.
The October 9 vote comes as the possibility of early parliamentary elections looms. The current ruling leftist coalition government opposes joining NATO.
An October 2014 opinion poll showed that a plurality of Swedes support NATO membership amid rising tensions with Russia.
The government says Russian submarines may have violated Swedish territorial waters repeatedly in the last year, and Swedish jets have frequently intercepted Russian fighters over the Baltic Sea.
Sweden has been a member of NATO's Partnership for Peace program since 1994 and is described on NATO's website as among the alliance's "most active partners."