Pressure has also mounted on independent media and human rights activists, with authorities labeling them "foreign agents" and "undesirable organizations."
The election is crucial because the Kremlin wants complete control over the State Duma, as the lower house of parliament is known.
However, an independent pollster, the Levada Center, puts United Russia's popularity as low as 27 percent, down from 44 percent in December 2015.
Opposition politicians and political analysts say Sunday's parliamentary election is President Vladimir Putin's latest bid to hold power. The Duma chosen this month will still be in place in 2024 when Putin's current term expires and he faces re-election.
Authorities mounted a fierce crackdown after protests erupted in January following the imprisonment of opposition leader Alexey Navalny. That followed his return from Germany after recovering from an assassination attempt that he blamed on the Kremlin.
No one from Navalny's team is running. Many have left the country after a new law barred those with ties to "extremist organizations" from standing for office.
Despite the crackdown, Navalny's team still plans to deploy its Smart Voting strategy -- a project to support candidates who are most likely to defeat those from United Russia.
Another prominent Kremlin critic, former lawmaker Dmitry Gudkov, has fled to Bulgaria after planning to run in a Moscow district against a United Russia candidate.
Authorities also jailed Andrei Pivovarov of the Open Russia opposition group financed by Russian tycoon and Putin critic Mikhail Khodorkovsky.
Yulia Galyamina, a prominent politician, convicted over her involvement in anti-Kremlin protest rallies, has also been barred from running.
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