Mitt Romney, mired in a frustrating slog toward the presidential nomination, has directed his top advisers to launch a multi-pronged effort to unite the Republican Party as the primaries draw to a close.

While Romney’s public activities are geared almost entirely toward winning the nomination, his campaign is on a private mission aimed beyond spring. Senior aides and surrogates — hoping to seize a moment when even some unfriendly Republicans are beginning to see Romney as inevitable — have spent the past several weeks making calls and visits to conservative leaders and activists who have resisted Romney’s candidacy.