Putin's nationally televised address Thursday came at the end of a campaign that has seen the Kremlin's dominance of the airwaves and sustained harassment of the opposition.
A landslide United Russia victory would give Putin a popular mandate, allowing him to be regarded as a "national leader" even after he steps as president down in May.
Seeking to mobilize apathetic voters, Putin cast Sunday's election as a stark choice between Russia's continuing revival and a plunge back to the economic meltdown and political turmoil of the 1990s.
"Don't, please don't think that the direction and pace of our development will be automatically maintained - that's a dangerous illusion," he said in the televised address, with a Russian flag behind him.
"Let's remember where we started eight years ago, the pit we had to dig the country out of," he said, pointing at economic achievements and efforts to pacify Chechnya.
The statement, broadcast repeatedly, came at the end of a campaign in which officials have thrown their full support behind United Russia and relentlessly hounded the opposition.
Nationwide television stations, all under state control, have given lavish positive coverage to United Russia and run programs castigating its rivals.
Putin's campaign speeches have consistently topped the national news, while representatives of other parties have been relegated to short advertising slots that attracted little public attention.
Last weekend, the official state Rossiya television station aired a documentary describing Russia's liberals as U.S. agents working to stage a popular uprising similar to those that helped pro-Western leaders take office in Ukraine and Georgia.
During the campaign, Putin sought to encourage a high turnout and solid support for United Russia by denouncing his political foes as "foreign-fed jackals" and lashing out at the West.
He accused Washington this week of trying to undermine the credibility of the vote by encouraging OSCE observers to boycott the election - a claim U.S. officials denied.
Opposition groups saw their rallies disbanded by riot police and scores of activists, including world chess champion Garry Kasparov, arrested. Kasparov was released Thursday after five days in jail.
Public sector workers including teachers and doctors have complained of being pressured to get absentee ballots and vote at their place of work - as part of a campaign by state apparatus to promote United Russia.
By leading the party's ticket, Putin has essentially turned the vote into a referendum on his rule. United Russia controls the current parliament and is expected to strengthen its domination by winning about 80 percent of seats, according to opinion polls.
The Communist Party is the only other party seen as certain to overcome the 7 percent threshold to make it into parliament.
It has remained unclear what position Putin might take after stepping down. He has said that an overwhelming victory by United Russia would give him the "moral right" to make sure the government continues to follow the course he has set.