Straujuma, accompanied by her young grand-daughter - arrived at Jaunmarupe school just after 1100 to cast her ballot.
Speaking to reporters outside she said: "I expect Unity to be first and I expect Harmony's support to drop because national security is a crucial issue... I can't support the [Harmony] view that Vladimir Putin is the best President for Russia."
On her appetite for retaining the premiership she was guarded, saying: "The main thing is to get into parliament and then we'll see."
An hour later Usakovs voted at council offices in the north of Riga. after waiting in line, he cast his ballot to the flash of cameras and talked to reporters outside in three languages.
"We expect to be in first place... It's good there are lines, as it means voter activity will be high," Usakovs said.
He rejected Straujuma's claim that national security would impact the Harmony vote.
"The country would definitely be safe in Harmony's hands and if any party should know about falling support it is Unity... Harmony is interested in pragmatic relations with all of its neighbours including Russia," Usakovs said.
Voting in Drabesi near Cesis, President Andris Berzins said the issues of national security and economic development were at the forefront of his mind.
"It seems to me that internal stability, peace and, of course, stable economic growth are the most important things," he told LTV.
According to figures from the Central Election Commission, by 1600 41% of voters had turned out.