A growing number of oil-market watchers say voters riled by soaring fuel costs may face far worse this summer, as factors ranging from unrest in Nigeria to slumping production in Russia could shove benchmark oil prices over $150 a barrel.

U.S. benchmark crude notched another record Tuesday, settling at $121.84 on the New York Mercantile Exchange. Nymex crude oil so far this year is up 27% and is now 17% above its previous inflation-adjusted record in April 1980. It is up 96% from a year ago.

Oil's...