HIMEJI, Japan — Campaigning for the July 21 parliamentary election began across Japan on Thursday, with opinion polls forecasting major gains for Prime Minister Shinzo Abe and his economic revival plan.

At stake in the election are half of the seats in the upper house of Parliament, where his main opponents, the Democratic Party, now have control. Mr. Abe’s conservative governing party, the Liberal Democrats, soundly defeated the Democrats in December in elections for the more powerful lower house of Parliament, and the polls so far suggest that they will repeat that feat in the upper house. If they succeed, Mr. Abe will be the first Japanese prime minister in years to break a rapid cycle of rise and fall for the country’s leaders.

Control of both houses of Parliament would give Mr. Abe more freedom to push forward a doctrine that his party now proudly calls Abenomics, a cocktail of monetary stimulus, government spending and promised economic changes meant to jolt Japan out of its long deflationary slump.

Enthusiasm for Abenomics has already driven up prices on the Tokyo stock market by one-third this year, and Japan’s aggressive easing of monetary policy has lifted the country’s mainstay exports by pushing down the yen's value against the dollar and other currencies. Japan's economy is now the fastest-growing among the Group of 7 major industrialized nations.