WATERLOO, Iowa — This ought to be Jeb Bush’s time.

He laid out a detailed plan to crush the Islamic State three months before the Paris terrorist attacks. He gave a well-received speech on rebuilding America’s military at the Citadel two weeks ago. He has shown new confidence discussing terrorism, the Syrian refugee crisis and American values, and how they intertwine.

But instead of winning a second look from voters, Mr. Bush continues to be ignored, and to languish in the polls, even as the nation contends with what he calls “serious times that require serious leadership.”

In an appearance at an Iowa military museum here on Tuesday, where a World War II-era P-51 Mustang was suspended overhead, the stage was set for Mr. Bush, the former Florida governor, to highlight his credentials to be commander in chief. He detailed his positions on defeating the Islamic State, and invoked his father’s waging of the 1991 Persian Gulf war.

Yet the crowd of 150 mostly older voters was scarcely stirred to applaud.

That has been typical of Mr. Bush’s rallies this year: His sober competence seems no match for the wild rhetorical excesses of Donald J. Trump, or for the base-stirring fervor of Senator Ted Cruz of Texas, whose Iowa rallies this week drew larger, more enthusiastic crowds than Mr. Bush attracted.