The deepening Midwest recession has heightened Republican concern about two incumbent senators earlier thought to be in good shape for reelection. Recent private polls show that David F. Durenberger (R-Minn.) has slipped almost into a dead heat with Democrat Mark Dayton, the free-spending department store heir and challenger.

Republican strategists now rate Durenberger perhaps their most vulnerable incumbent but hope to turn the race around by publicizing Dayton's campaign expenditures.

Meanwhile, a poll published Thursday by The St. Louis Globe-Democrat showed Sen. John C. Danforth (R-Mo.) with a 56-to-39 percent lead over state Sen. Harriet Woods. But the poll noted that, because so much of Danforth's support is "soft," the two are virtually even in committed strength.

Missouri Gov. Christopher S. Bond (R), upset by a Democrat after similar polls in 1976, has warned Vice President Bush that Danforth needs to toughen his campaign and focus on economic concerns.

Republicans say both incumbents are popular, and attribute their weakness to Democrats' reverting to party-line voting because of tough economic times. The same factor is causing Republicans to downgrade their chances of holding governorships in Minnesota and Wisconsin, where Democratic primaries in mid-September produced surprise nominees and briefly raised their hopes.