Sweden is now sharply divided over big, emotive issues like immigration and the future of the welfare state. And after two terms of center-right government and market-oriented policies, the electorate’s center of gravity appears to be shifting back toward the center-left and the Social Democrats, who dominated the country for 80 years before 2006.

Each side warns about the fringe bedfellows that the other might have to depend on to form a government after a close result on Sunday. The left suggests that Mr. Reinfeldt might try to cling to power by allying with the far-right, anti-immigrant Sweden Democrats, who are polling around 10 percent, a reflection of growing dissatisfaction over the country’s long-held posture of welcoming refugees.

The right, in turn, warns that a government led by the Social Democrats, who have promised to raise taxes to provide more money for schools and welfare programs, would be dependent on support from the former communists of the Left Party, who make no secret of their opposition to private profit in the public sector.

“Business has much to fear from a red-green administration,” said Anna Kinberg Batra, parliamentary leader of Mr. Reinfeldt’s party, the Moderates, in arguing that a left-leaning coalition could unravel the country’s economic progress.

Jacob Wallenberg, scion of the family whose foundations own huge stakes in Swedish industry — close to 40 percent of the total value of the Swedish stock exchange — has warned that a “massive shift to the left” could prompt entrepreneurs to flee the country.