Republican presidential candidate Gov. Mitt Romney speaks in Washington.

By JEFF ZELENY and JIM RUTENBERG

c.2012 New York Times News Service

WASHINGTON - Mitt Romney, having 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 Barack Obama's stewardship of the economy.

Romney, who formally secured the Republican presidential nomination on Tuesday by winning a final batch of necessary 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 Romney's tenure as head of Bain Capital, which Obama has portrayed as evidence that Romney would not look out for the middle class.

Romney was dogged Tuesday by his decision to attend a Las Vegas fundraiser with Donald J. Trump, who spent much of the day in television interviews pressing the long-discredited view that the president is not a U.S. 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 he would be deeply wounded by his combative primary. For a candidate who is not naturally beloved by many conservatives, Romney has faced little resistance unifying party activists, donors and elected officials who want to keep Obama from winning a second term.

With the party quickly coalescing behind him, 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, Romney participated in a fundraising call with Gov. Rick Perry of Texas, whose formidable network of donors had been a source of concern to Romney's team during the primary season. Perry, who had vigorously questioned Romney's ability to beat 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," 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."

Romney still faces a dual challenge of overcoming tepid favorability ratings and persuading voters why they should support him and not simply vote against 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 Romney.

"They've had a hard time trying to paint Gov. 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 Obama's team in Chicago.

Following a run of television advertisements and Web videos attacking Romney's work at the private equity firm Bain Capital, most major polls show 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.

Romney's campaign has been particularly buoyed by surveys showing that he is competitive with ObamaB 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 Obama's efforts to undercut that perception by sowing doubts about Romney's work at Bain and his claims that he created tens of thousands of jobs.

But they are hoping that his strong ratings on the economy will help them to meet their main imperative before the party conventions: to solidify perceptions of Romney as an adequate replacement for Obama among key swing voters who like the president but are ready to "fire him" - as one aide put it - if they can bring themselves to trust 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 Obama, "very, very impressive." Though Richardson said he believed Romney's stance on immigration would hobble him with Hispanics in key 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, Obama's aides say they are unfazed by 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 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 Virginia, North Carolina, Ohio and Florida and then picking up one "wild card" state from a group that includes New Hampshire, Iowa, Nevada, Colorado, Wisconsin and Michigan.

For his part, 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 here on Commercial Street, strategists are now running the campaign they began preparing for not long after 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 Obama and Romney in recent weeks have overshadowed another challenge that 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 Romney's finance team. "But I never felt this good about our chances four years ago."