OXON HILL, Md. — The largest annual gathering of Republican activists began here Thursday with appearances by rival presidential hopefuls offering their party starkly different paths back to prominence — and diagnoses of what ails it — after last fall’s demoralizing losses.

“We don’t need any new ideas,” Senator Marco Rubio of Florida told a room packed with cheering grass-roots activists, anticipating what he predicted would be liberal critiques of his remarks. “The idea is called America, and it still works.”

Speaking immediately after him, Senator Rand Paul of Kentucky declared, “The G.O.P. of old has grown stale and moss-covered.” A “new G.O.P.,” he said, “will need to embrace liberty in both the economic and the personal sphere.”

The yearly assembly of the Conservative Political Action Conference, or CPAC, is a showcase of the Republican Party’s top presidential prospects, whose reception before a crowd of critical, future primary voters and volunteers is watched carefully by party leaders, donors and news media handicappers.