(Mr. Reagan failed to win the Republican nomination in 1968 and 1976 before being elected president in 1980.)

Even as they defend a potential candidacy, Mr. Romney’s supporters underscore the gulf between grass-roots activists and the affluent network of donors, many of whom are business executives like him and are eager for him to undertake another campaign. The activists include conservatives who view Mr. Romney as too moderate and others who dislike him because they do not think he can win.

Asked what would be different in a third campaign, Mr. Fehrnstrom said, “Of all the potential candidates looking at the race, only Mitt Romney has proven that he can raise the billion dollars or more that would be needed, and he is the only one who is a proven vote-getter in a national race.”

Many early polls, which lean heavily on name recognition, show Mr. Romney leading the field when his name is included, a trend, one adviser said, that contributes to Mr. Romney’s temptation. Part of that strength stems from his having led the ticket in 2012. But Mr. Fehrnstrom, in a barely veiled reference to former Gov. Jeb Bush of Florida, noted, “You’ve got two candidates with very high name identification.”

Mr. Romney and his lieutenants have been hastily contacting previous supporters, and even sounding out potential campaign managers, over the phone since the former Massachusetts governor revealed Friday to a group of contributors that he was considering another White House bid. A number of former aides and early backers have since indicated that they would support him again.

But not a single current Republican senator has come forward to state support for the party’s 2012 standard-bearer; three of them, separately, skirted the question the same way when asked about Mr. Romney’s prospects: “The more the merrier.”

Part of this owes to the reluctance of a politician to wager on somebody who has not even said he would run. With other potential candidates like Mr. Bush likely to enter the race, few elected officials want to commit this early. But the apprehension toward another bid by Mr. Romney extends beyond that.