Romney, Sealing Nomination, Steps Up Attack on Obama Justin Sullivan/Getty Images Mitt Romney greeted supporters, including one considerably shy of voting age, at a campaign rally Tuesday in Craig, Colo. WASHINGTON - Mitt Romney, having initially weathered the first sustained general election attack, is entering a critical 90-day stretch to the Republican convention on relatively equal footing with the White House and is unleashing a new offensive to win over independent voters and further undermine confidence in President Obama's stewardship of the economy. Mr. Romney, who formally secured the Republican presidential nomination on Tuesday by winning delegates in the Texas primary, introduced a new line of criticism: accusing the president of squandering taxpayer money on companies like Solyndra, which declared bankruptcy last year after receiving $528 million in federal loan guarantees. It amounted to a counterpunch to the White House's assault on Mr. Romney's tenure as head of Bain Capital, which Mr. Obama has portrayed as evidence that Mr. Romney would not look out for the middle class. Mr. Romney was dogged Tuesday by his decision to attend a Las Vegas fund-raiser with Donald J. Trump, who spent much of the day in television interviews pressing the long-discredited view that the president is not a United States citizen. The Romney campaign rejected that view and conceded it was an unnecessary distraction. But the resilience of the Romney campaign, at least in the first six weeks since he emerged as the party's likely nominee, has proved frustrating to some Democrats who predicted that he would be deeply wounded by the combative primary race. For a candidate who is not naturally beloved by many conservatives, Mr. Romney has faced little resistance unifying party activists, donors and elected officials who want to keep Mr. Obama from winning a second term. With the party quickly coalescing behind him, Mr. Romney met in Las Vegas on Tuesday with Sheldon Adelson, the casino magnate whose family provided $20 million to the "super PAC" that supported Newt Gingrich. Last week, Mr. Romney participated in a fund-raising call with Gov. Rick Perry of Texas, whose formidable network of donors had been a source of concern to Mr. Romney's team during the primary season. Mr. Perry, who had vigorously questioned Mr. Romney's ability to beat Mr. Obama, said his former rival had emerged stronger from the primaries. "He's a better candidate today than he was four years ago and four months ago," Mr. Perry said in an interview. He has shown improvement all along the way, and that is always the sign of a good competitor who continues to take lessons learned. Mr. Romney still faces a dual challenge of overcoming tepid favorability ratings and persuading voters why they should support him and not simply vote against Mr. Obama. Even though Election Day is five months away, strategists in both campaigns see the phase from Memorial Day to Labor Day as crucial, particularly for defining Mr. Romney. "They've had a hard time trying to paint Governor Romney as somehow sinister," said Ed Gillespie, a senior adviser to the Romney campaign. The fact is every time they attack Mitt Romney for his experience in the private sector, they reinforce the idea that President Obama is hostile to the private sector. At the Romney campaign headquarters in Boston, his advisers exuded a palpable sense of relief and a growing air of confidence after surviving the first phase of artillery from Mr. Obama's team in Chicago. Following a run of television advertisements and Web videos attacking Mr. Romney's work at the private equity firm Bain Capital, most major polls show that the two candidates are statistically tied. While polls at this stage of an election can be unreliable indicators of the outcome in the fall, they underscore that the race is likely to be close. Mr. Romney's campaign has been particularly buoyed by surveys showing that he is competitive with Mr. Obama when voters are asked whom they trust most to handle the economy. His aides said in interviews that they were keeping a wary eye on Mr. Obama's efforts to undercut that perception by sowing doubts about Mr. Romney's work at Bain and his claims that he created tens of thousands of jobs. But they are hoping his strong ratings on the economy will help them meet their main imperative before the party conventions: to solidify perceptions of Mr. Romney as an adequate replacement for Mr. Obama among swing voters who like the president but are ready to "fire him" - as one aide put it - if they can bring themselves to trust Mr. Romney. In an interview, former Gov. Bill Richardson of New Mexico, a Democrat, called the Romney campaign's "rapid response" operation - tasked with conducting the minute-by-minute fight against Mr. Obama - as "very, very impressive." Though Mr. Richardson said he believed that Mr. Romney's stance on immigration would hobble him with Hispanics in important swing states like Nevada and Colorado, he added, "We've got to fight for every vote." "I am amazed at how rapidly the Republican Party has united around Romney," he said. I thought it would take longer. In interviews, Mr. Obama's aides say they are unfazed by Mr. Romney's quick rise to campaign parity, arguing that he still faces a challenge in reversing a primary season polling trend in which more people viewed him unfavorably than favorably. The Romney campaign is still expanding its operation for the fight with Mr. Obama, who has been building a team for more than a year in battleground states. A presentation made to top Republican donors last week in Boston shows that the campaign is placing a priority on winning Florida, North Carolina, Ohio and Virginia and then picking up one "wild card" state from a group that includes Colorado, Iowa, Michigan, Nevada, New Hampshire and Wisconsin. For his part, Mr. Romney is trying to get voters to envision him as president. His debut television advertisements present what a "President Romney" would do on "Day 1," with an announcer saying: "President Romney issues order to begin replacing Obamacare with common-sense health care reform - that's what a Romney presidency would be like." At Romney headquarters in Boston, strategists are now running the campaign they began preparing for not long after Mr. Obama took office. They have extensively studied his record and for the next several weeks intend to raise issues, like Solyndra, in hopes of putting the president's team on defense. The increasingly intense exchanges between Mr. Obama and Mr. Romney in recent weeks have overshadowed another challenge that Mr. Romney has seemed to overcome: his fellow Republicans are no longer complaining about him or questioning his conservative record. "I was very involved in the McCain campaign, and I love McCain a lot," said Brian Ballard, a Florida contributor who is a co-chairman of Mr. Romney's finance team. But I never felt this good about our chances four years ago.