This presidential race may not be big enough for the two of them.

Interviewed on "With All Due Respect" Monday, former Mitt Romney advisor Dan Senor said he believes that his former boss will sit out the 2016 presidential race if and when former Florida Governor Jeb Bush declares himself a candidate. 

"My general sense of it is that the theory of Romney possibly getting in is if we had a crack-up in the Republican primary, a real crack-up, the field blew up, there was no consensus candidate that can unite the party," Senor told hosts Mark Halperin and John Heilemann."That was, by the way, a real possibility because the sort of hard right candidates—Rand Paul, Ted Cruz—were clearly running and the mainstream candidates either had obstacles or were hedging. I think these moves that Jeb makes, and others are going to start making, are going to start turning that upside down. They're going to stop the hedging and jump right in."

Earlier in the show, Halperin asked Senor about a recent Bloomberg Businessweek story comparing the investment portfolios of Romney and Bush, and whether the latter's business dealings could prove a political liability in the same way those issues did for Romney. 

"Jeb is not running on his private equity career. He's running on having been one of the most successful conservative governors in the country. He happens to have ties to the financial industry, so does Hillary Clinton, so does Governor Christie," Senor said. 

Over the weekend, Bush announced he was releasing approximately 250,000 emails from his time in office in Florida, and would also publish and e-book on policy, two more signs that Senor may be right about at least one former governor's plans.

"They exist in the same lane, in the same bracket in the Republican primary. You know, mainstream, center-right candidates. Christie was the most competitive, biggest player in that lane, and Jeb is possibly entering it," Senor said.

A Marist poll released Monday showed why Bush should hope Romney does exit the field. If both candidates were to run for president, 19 percent of Republican voters, a majority, would choose Romney, while 14 percent said they would pick Bush. If Romney sits out the election, Bush tops the GOP field with 16 percent.