Remote Bhutan's line of "dragon kings" ceded absolute power five years ago, introducing democracy to an electorate of fewer than 400,000 people.
The royalist DPT stormed the first election by a landslide in 2008 and won this year's primary round with 45 percent of votes.
But recent gains by the PDP have shaken up the contest.
"The general perception is that it could be a neck and neck race, where every vote counts," said an editorial on the national Kuensel newspaper website on Friday, after a 48-hour campaigning blackout began.
The DPT party has won popularity with rural communities -- about 70 percent of the population -- for improving their access to roads, mobile phone networks and electricity in the past five years.
But the election process has been stirred up by