At a rally in Detroit Aug. 8, Republican presidential candidate Donald Trump outlined what he would do as president to take the U.S. economy to "amazing new heights." (Sarah Parnass/The Washington Post)

Seeking to put the most difficult stretch of his campaign behind him, Republican presidential nominee Donald Trump used a major economic speech Monday to reach out to two voting blocs that remain critical to his faltering chances of winning in November: traditional fiscal conservatives and disaffected blue-collar workers.

But Trump faced a new round of resistance from within his party that threatened to stall his effort to move beyond the uproar he caused last week. In an opinion column published by The Washington Post late Monday, Sen. Susan Collins (Maine) became the latest sitting Republican senator to declare that she will not support Trump. In addition, dozens of national security officials who served in GOP administrations signed a letter saying that he is “not qualified” to be president.

Reading from a teleprompter at the Detroit Economic Club and pausing calmly when protesters interrupted him, Trump assailed Democratic rival Hillary Clinton and cast himself as the only change candidate on economic issues. He did so in part with tax-cutting, regulation-curbing plans that are squarely mainstream in his party and in part with his now-familiar attacks on the forces of globalization that have unnerved many workers. He took swipes at free-trade deals championed by GOP leaders and attacked immigrants and refugees.

The Republican nominee shared few new policy details and continued to offer no specifics for how he would pay for tax cuts or spending increases large enough to balloon the federal budget deficit. He promised more clarity in coming weeks.

Trump proposed a new set of individual income tax rates higher than he previously suggested, but he also promised to bring rates lower than they were even during the George W. Bush administration. He was vague in other areas, including a promise for major federal infrastructure spending and another, the only new policy proposal in the speech, that would allow working families to deduct child care-costs from their federal income taxes.

Throughout his address, Trump took sharp aim at Clinton. He held up Detroit, which has been devastated by manufacturing job losses, as “the living, breathing example” of her “failed economic agenda.”

“I want to jump-start America. It can be done. And it won’t even be that hard,” he said.

At a rally in St. Petersburg, Fla., Clinton assailed Trump’s plan as an outdated replica of previous Republican pitches, saying it would “give super-big tax breaks to large corporations and the really wealthy” and “basically just repackage trickle-down economics.”

She added: “You know that old saying, ‘Fool me once, shame on you. Fool me twice, shame on me.’ ”

Clinton framed rejecting Trump as a national imperative.

“Just imagine Donald Trump in the Oval Office, facing a real crisis,” she said. “What happens when somebody gets under his skin? I don’t know if the United States can afford that kind of risk.”

On income taxes, the business mogul said he would work with House Republicans to implement the three brackets they have proposed: 12 percent, 25 percent and 33 percent. The move puts Trump in line with Speaker Paul D. Ryan (Wis.), with whom Trump has had a tense alliance.

Previously, Trump proposed tax brackets of 0 percent, 10 percent, 20 percent and 25 percent. He continued to call for a 15 percent corporate income tax rate for all businesses, which is lower than Ryan’s proposed 20 percent corporate rate.

Trump also promised to end some “special interest” tax breaks but named only one, the “carried interest” provision that many investment fund managers use to reduce their tax liability. Experts cautioned, though, that Trump’s plan would still deliver a windfall to such investors, because it would reduce income and corporate rates.

Lacking more details from the campaign, it is difficult to say how much Trump’s revisions to his tax rates would alter the cost of his economic plan, which analysts had previously estimated could reduce federal revenue by $10 trillion over the next decade. Equally difficult to measure are the benefits the plan would deliver to taxpayers across income levels.

“It seems very likely that this version of the plan will lose less revenue than the last version” because it will contain relatively smaller tax cuts for individuals, Scott Greenberg, an analyst at the nonpartisan Tax Foundation, said in an interview. A key question, he added, is the level at which various marginal tax rates begin to take effect: “A tax plan where the 33 percent [rate] kicks in at $250,000 and one where it kicks in at $750,000 are two very different tax plans.”

Trump’s economic focus followed a week in which he stoked tensions with party leaders by initially declining to endorse Ryan and Sen. John McCain (Ariz.) in their primaries this month. Trump also drew widespread ire for criticizing the Muslim parents of a U.S. Army captain who was killed in Iraq, and he fell dramatically behind Clinton in public polls.

The measured, pre-written remarks in Detroit on Monday were intended to steady a listing campaign, and conservatives received the calls for tax and regulation cuts positively. But most reaction to the speech, even among conservatives, was mixed. In the end, Monday served as a reminder that many Republicans remain highly skeptical of Trump.

Lanhee Chen, who was the policy director for GOP presidential nominee Mitt Romney in 2012, called the speech a “mixed bag.” He praised the details on taxes as a nod to more orthodox Republican views, but he said Trump’s main challenge now is to prove to voters that he is as serious about policy-making as he is about picking fights with critics.

“One speech is not going to change a narrative,” Chen said.

Also Monday, a group of 50 former national security officials who served under Republican presidents signed a letter warning that Trump “would be the most reckless President in American history.”

The letter was signed by Michael Chertoff and Tom Ridge, former secretaries of homeland security; Michael V. Hayden, a former director of the CIA and the National Security Agency; and John D. Negroponte, a former director of national intelligence and deputy secretary of state, among others.

Separately, Wadi Gaitan, the chief spokesman for the Florida Republican Party, announced that, as a result of differences with Trump, he is leaving his job to join a conservative organization.

“I’m thankful for my almost two years with the Florida GOP, however, moving on gives me a great, new opportunity to continue promoting free market solutions while avoiding efforts that support Donald Trump,” Gaitan, who is Hispanic, said in a statement.

In Detroit, protesters sidetracked Trump’s speech repeatedly. Unlike in large rallies where Trump often calls for demonstrators to be removed, riling up the crowd, he waited patiently as they were escorted out Monday. At one point he remarked calmly, “This is all very well-planned out.”

His speech outlined a plan designed to accelerate economic growth, largely in classic conservative fashion: by reducing taxes and regulations on businesses and by opening vast new swaths of federal land and water to drilling. He said that as president, he would sign an executive order creating a temporary regulatory moratorium on new agency regulations.

“I am going to cut regulations massively,” he said. “Massively.”

Freezing all pending federal regulations would include many Wall Street regulations created by the Dodd-Frank legislation passed in the wake of the financial crisis. Trump’s energy agenda would open new sections of American coastal waters to offshore oil drilling and sweep away the Obama administration’s efforts to fight climate change. Both moves have frequently found widespread support among Republican lawmakers and in conservative policy circles.

In other areas, he skirted or defied Republican orthodoxy. Trump made no attempt to propose spending cuts or other measures to offset his proposed tax-rate cuts or begin to reduce the national debt, as he has promised to do in the past. His child-care expense plan could increase the debt even further, unless it were offset by spending cuts or a rapid increase in economic growth. So could an infrastructure spending plan that he has said could cost more than $500 billion.

The plan also promises to increase growth by reducing the United States’ trade deficit with China and other trading partners, in part by levying tariffs on imported goods from those countries. Some economists, including Trump adviser Peter Navarro, say that reducing the trade deficit would boost growth. Others, including Mark Zandi of Moody’s Analytics, warn that a tariff war could push the United States and much of the world into recession.

Trump has adopted hard-line opposition to sweeping trade agreements, arguing that they have hurt American workers. He reiterated his commitment to renegotiating the North American Free Trade Agreement and withdrawing from the Trans-Pacific Partnership. He singled out President Bill Clinton for signing NAFTA and accused the accord of moving U.S. jobs abroad. He said a vote for Hillary Clinton is a vote for TPP.

“The one common feature of every Hillary Clinton idea is that it punishes you for working and doing business in the United States,” Trump said.

As secretary of state, Clinton praised TPP. But in the Democratic primary, she abandoned her support for the agreement. Most congressional Republicans support the multi-nation pact.

Anne Gearan in St. Petersburg, Fla., and Carole Morello and Ed O’Keefe in Washington contributed to this report.