“Juleanna Glover, a Republican strategist backing Mr. Bush, said it was ‘totally predictable’ that campaigns have ups and downs. She said that when voters come closer to making a decision, they would look at the three candidates who were credible presidential nominees — Mr. Bush, John Kasich, the Ohio governor, and Mr. Rubio — and not Mr. Trump. She said Mr. Bush’s low poll numbers were similar to Mr. Rubio and Mr. Kasich, underscoring how the mainstream candidates were being drowned out by the current frontrunners.”