Republican National Committee Chairman Reince Priebus marked the transition Wednesday by proclaiming Romney the party's "presumptive nominee."
Romney's campaign also appointed several senior staff members to work on an informal takeover of the committee's national infrastructure.
"We will ensure that our finance, political and communications teams are fully synchronized," Priebus said. "I am excited that these two top-notch operations will start to integrate and present a unified team to defeat Barack Obama."
At the same time, fading Republican contender Newt Gingrich signaled that he would likely follow Santorum out of the race and called on the GOP to unite behind Romney. Aides confirmed that Gingrich will leave the race next week and said he was likely to endorse his one-time rival.
At his victory celebration Tuesday night following a sweep of five northeastern primaries, Romney blasted Obama as a man whose time in office has been marked by "false promises and weak leadership" in a time of economic struggle.
"Tonight is the start of a new campaign," the former Massachusetts governor declared.
Tuesday's contests were the first since Santorum concededthe race, and the former Pennsylvania senator said he intends to sit down with Romney's representatives Wednesday and Romney himself in the next week or two.
"Mitt Romney is going to be the nominee," Santorum told CNN, "and I'm going to support the nominee."
Gingrich also said he expects Romney will be the nominee and he called on the party to unite behind the former Massachusetts governor. But at a North Carolina appearance Wednesday, Gingrich stopped short of endorsing Romney and left his future unclear.
"You have to at some point be honest about what's happening in the real world as opposed to what you would like to have happened," Gingrich told supporters.
Gingrich and Santorum have aggressively questioned Romney's conservative credentials in recent months. Santorum said last month that Romney is the worst candidate to face Obama. But Tuesday night, asked on CNN if Romney was "the right guy" to represent the Republican Party, Santorum said he was.
Romney, meanwhile, was privately intensifying his fundraising efforts Wednesday and Thursday to prepare for what may be the most expensive presidential contest in the history of American politics. He exuded confidence Tuesday night, but he's facing a 10-to-1 cash disadvantage in a general election matchup against the Democratic president.
The presumptive Republican nominee has at least six closed-door fundraising events in two days in New York and New Jersey. They may be among his final private meetings with donors, according to campaign officials who confirmed that Romney would begin to open some finance events to reporters as early as next week. The officials requested anonymity to discuss internal decisions.
Lifting the curtain on what has been a private process for months would come less than 10 days after reporters outside a Palm Beach, Fla., fundraising event overheard Romneysharing previously undisclosed details about his tax plan. The episode was an embarrassment for Romney, who has been facing growing calls for transparency in his role as the GOP's likely candidate.
While the ground rules have yet to be finalized, one campaign official said Romney would probably begin inviting a small group of reporters into larger fundraising events — particularly those in which the candidate offers remarks — in the coming week. That's largely the same policy Obama follows.
Romney's success will depend, at least in part, on his ability to compete with Obama's bank account.
Romney's campaign had only about $10 million in the bank at the end of March, according to federal filings. All told, Obama reported more than $104 million in his account, having already spent nearly $90 million on the general election. Election Day is Nov. 6.
Romney was eager to turn the political page after Tuesday's primary wins in New York, Pennsylvania, Connecticut, Rhode Island and Delaware.
"After 43 primaries and caucuses, many long days and not a few long nights, I can say with confidence — and gratitude — that you have given me a great honor and solemn responsibility," he told supporters gathered in New Hampshire. He urged all who are struggling in a shaky economy to "hold on a little longer — a better America begins tonight."
Obama set the modern fundraising record in 2008, when he and his Republican rival, Sen. John McCain, spent more than $1 billion combined — with Obama spending more than $730 million. In 2004, the two major-party candidates set a record of $700 million.
Obama opened his finance events to press coverage in June 2008, shortly after becoming his party's presumptive nominee. As president, he largely plays by the same rules. If he makes remarks during the event — no matter how big or small — the press is allowed in.
The Romney campaign has refused to provide the specific times and locations of this week's fundraising events.
The process of formally integrating his campaign with the RNC had already been under way.
Three Romney advisers spent two days at an annual state party gathering in Arizona last week to start laying the groundwork for the effort. Longtime Romney confidante Ron Kaufman, also an RNC member, organized that effort and will continue to serve in such a role.
Republican operative Brian Jones, a veteran of McCain's 2008 presidential campaign, will take the lead on coordinating between the two entities. Kevin Madden, who was Romney's spokesman in 2008, will advise the communications team.