Mitt Romney sought to use the coveted endorsement of Jeb Bush on Wednesday to amplify his call for Republicans to rally behind his candidacy and get on with the mission of ousting President Obama.
As Mr. Romney savored a decisive victory in the Illinois primary on Tuesday over Rick Santorum, he learned that Mr. Bush — whose family name and appeal to both conservatives and moderates makes him as much an embodiment of the Republican establishment as anyone — was declaring his support and urging Republican voters to follow him.
The announcement provided Mr. Romney with additional ammunition to persuade other party leaders and constituencies that his nomination is all but inevitable and seemed to close off efforts by restless Republicans to coax Mr. Bush into the race.
“Now is the time for Republicans to unite behind Governor Romney and take our message of fiscal conservatism and job creation to all voters this fall,” Mr. Bush said, calling him “a leader who understands the economy.”
But if the endorsement held the potential to further choke off the oxygen to Mr. Santorum’s insurgent candidacy, the Romney campaign inadvertently gave Mr. Santorum a new supply when a senior adviser went on CNN and seemed to suggest that Mr. Romney’s conservative positions in the primary season could change like an Etch a Sketch drawing.
“I think you hit a reset button for the fall campaign,” Eric Fehrnstrom, a longtime adviser to Mr. Romney, said in response to a question about pivoting to a matchup with Mr. Obama and appealing to moderate swing voters. “Everything changes. It’s almost like an Etch a Sketch. You can kind of shake it up and restart all over again.”
The reference to a children’s drawing toy that erases the last image with a simple shake immediately fed attacks from Mr. Romney’s rivals that he was an untrustworthy standard-bearer for the conservative cause.
“You take whatever he said and you can shake it up and it will be gone and he’s going to draw a whole new picture for the general election,” Mr. Santorum said while campaigning in Louisiana, which votes Saturday. “Well that should be comforting to all of you who are voting in this primary.”
Later, Mr. Santorum’s spokeswoman, Alice Stewart, handed Etch a Sketches to reporters covering Mr. Romney’s town hall-style meeting in Arbutus, Md.
In a Twitter message Newt Gingrich wrote, “Etch a Sketch is a great toy, but a losing strategy.”
The comments from Mr. Fehrnstrom followed a Romney campaign pattern of committing unforced errors after major victories. And they flared throughout the day from Republicans and Democrats alike, moving beyond politics into a hard-to-forget moment of popular culture that could be difficult for Mr. Romney to shake.
Even Rush Limbaugh questioned whether the remark was an error or a deliberate campaign strategy to send the message, “Don’t worry, we’ll do the right things to get the moderates.”
The Romney campaign at first dismissed the criticism. At an afternoon rally in Maryland, Mr. Romney initially swatted away reporters who tried to pose questions, but later said his adviser simply was referring to the mechanics of a larger campaign.
“The issues I’m running on will be exactly the same,” Mr. Romney said. “I’m running as a conservative Republican, I was a conservative Republican governor, I’ll be running as a conservative Republican nominee — or, excuse me, at that point, hopefully, nominee for president.”
The dustup competed with Mr. Romney’s intensified efforts on Wednesday to bring new and not-so-subtle pressure on Mr. Santorum to drop his candidacy, arguing that his chances were fading with Mr. Romney’s widening lead in the delegate and popular vote counts and endorsements from the likes of Mr. Bush.
But they are treading carefully and with the full knowledge that Mr. Santorum has no plans to step aside now and that to push him too far is to risk backlash from his ardent supporters in upcoming primary states.
“He is smart enough to know that to have a future in the party, for a role in the government, or to run again or whatever, there will come a time when he needs to be a team player,” Charlie Black, an informal adviser to Mr. Romney and a veteran Republican presidential strategist, said of Mr. Santorum.
But, he said in an interview, “We’re not there yet.” He added, “We’re halfway through the process, and he can still see a scenario by which he may win, even if an objective person may say that’s sort of threading the needle.”
Mindful of its tone, a directive was sent from the campaign’s headquarters in Boston telling aides and advisers not to openly urge the other Republican candidates to get out of the race. For now, an adviser said, it was probably better for Mr. Romney to have three opponents splitting votes among themselves rather than having a singular alternative to Mr. Romney.
Yet senior Republican advisers in governors’ offices across the country and on Capitol Hill said Wednesday that the tide was unmistakably turning toward Mr. Romney, even though levels of enthusiasm were mixed. Mr. Romney was scheduled to attend a fund-raiser on Thursday to try and improve his connection with Congressional Republicans.
Russ Walker, a vice president of Freedomworks, an influential Tea Party organization that has been critical of Mr. Romney, said Wednesday that Mr. Romney was moving closer to winning the Republican nomination. He said he feared that a prolonged primary campaign would distract Republicans from Senate races.
But suggestions that Mr. Romney was on his way to becoming the nominee was not a sentiment shared by the campaigns of Mr. Santorum and Mr. Gingrich, who are urging conservative activists and party leaders against a rush to judgment. His rivals are not only not contemplating dropping out, but were taking steps to move forward.
As post-Illinois news media chatter began focusing on a “one-man race” and the “beginning of the end” for Mr. Santorum’s insurgent bid, Mr. Santorum’s campaign was vying for another Southern victory on Saturday in Louisiana, where both campaigns consider him to be leading.
He was also looking ahead to the April 24 primary in his home state of Pennsylvania, where he hopes to secure a major win on a day that several big Romney-leaning states vote, including New York, Connecticut and Rhode Island.
Mr. Santorum has also repeatedly worked to win the endorsement of governors and donors who had once backed Gov. Rick Perry of Texas. (He also has called Mr. Perry, two aides said, in hopes of nudging him away from his endorsement of Mr. Gingrich.)
At the same time, aides to Mr. Gingrich were scouting for office space in advance of Indiana’s primary on May 8, a sign they said was evidence of their commitment to stay in the race.