Correction Appended

WASHINGTON, Oct. 15 — Senior Republican leaders have concluded that Senator Mike DeWine of Ohio, a pivotal state in this year’s fierce midterm election battles, is likely to be heading for defeat and are moving to reduce financial support for his race and divert party money to other embattled Republican senators, party officials said.

The decision to effectively write off Mr. DeWine’s seat, after a series of internal Republican polls showed him falling behind his Democratic challenger, is part of a fluid series of choices by top leaders in both parties as they set the strategic framework of the campaign’s final three weeks, signaling, by where they are spending television money and other resources, the Senate and House races where they believe they have the best chances of success.

Republicans are now pinning their hopes of holding the Senate on three states — Missouri, Tennessee and, with Ohio off the table, probably Virginia — while trying to hold on to the House by pouring money into districts where Republicans have a strong historical or registration advantage, party officials said Sunday. Republicans also said they would run advertisements in New Jersey this week to test the vulnerability of Senator Robert Menendez, one of the few Democrats who appear endangered.

Senior national Republican strategists who had been briefed on decisions made during the party’s internal deliberations discussed the overall strategic thrusts but declined to provide specific dollar figures, saying that would give too much information to the Democrats.