(CNN) Donald Trump will have to do more than hold his tongue for a day and offer an olive branch to Paul Ryan to steady his faltering campaign.

But the Republican nominee is at least making a start.

After a week of brutal self-inflicted wounds, Trump moved to get back on track Monday amid mounting questions about his character and suitability for the presidency.

Leaving his unconventional political persona at home, Trump went to the Detroit Economic Club, a traditional venue where generations of presidential candidates have spoken. He laid out more substance on his economic plans than ever before and made a significant move toward conservative orthodoxy on tax policy in an apparent gesture to Ryan, the GOP House speaker with whom he feuded last week.

He also made a point of showing restraint when more than a dozen hecklers interrupted him -- a dramatic departure from the often-aggressive approach he's taken to protesters at his rallies.

His softer touch was so notable that his campaign later fired off a fundraising email in his name boasting that he stuck to the script.

"We stayed on message and delivered our positive platform to America," the fundraising appeal said.

Despite some upbeat indicators, Trump also got a barrage of reminders Monday about how much his campaign is now focused on damage control and a glimpse of the mounting impediments to victory in November.

Sen. Susan Collins of Maine became the latest high profile Republican to say she won't vote for her party's nominee.

"Donald Trump does not reflect historical Republican values nor the inclusive approach to governing that is critical to healing the divisions in our country," she wrote in an op-ed published in The Washington Post

GOP national security experts vs. Trump

In a stunning development, more than 50 Republican foreign policy and national security experts -- including many who worked for former President George W. Bush -- signed a letter denouncing Trump and refusing to vote for him.

"We are convinced that in the Oval Office, he would be the most reckless President in American history," the former officials, who included former CIA chief Michael Hayden, former Deputy Secretary of State John Negroponte and Eric Edelman, former Vice President Dick Cheney's national security adviser.

The letter, first reported by The New York Times , emerged days after President Barack Obama, in an unprecedented move for a sitting commander in chief, warned that Trump lacked the intellect and temperament to succeed him.

It also raised doubts about how many of the GOP's foreign policy elite would agree to fill diplomatic, defense and intelligence posts should Trump win in November.

In another unexpected development, the GOP nominee found out Monday he would face a general election headache from within the conservative movement.

Evan McMullin, a former CIA officer, plans to offer conservatives who abhor Trump someone to vote for. He's unlikely to pose a nationwide threat. But McMullin could prove a complicating factor in the solidly Republican state of Utah. McMullin, a practice Mormon, could jeopardize the Republican nominee's grip on a deep-red state -- and could prove damaging in a close race between Trump and Clinton."

Latest polling

And after a week of grim polling numbers, Trump had more bad news on Monday.

A Monmouth University survey found Clinton up 12 points nationally over Trump among registered voters. A CNN Poll of Polls survey, meanwhile, put the former secretary of state 10 points ahead.

The surveys reflect the bounce Clinton is still enjoying from her Democratic National Convention and could change once the race settles and voters pay more attention in the fall. Still, the data appear to reveal doubts about Trump triggered last week by his feud with the parents of fallen Muslim solider Capt. Humayun Khan, attacks on Republican Party leaders and his missteps on foreign policy.

Clinton is keeping the pressure on. Her campaign chairman, John Podesta, released a statement late Monday saying Clinton "looks forward to participating in all three presidential debates" that are scheduled in September and October.

"The only issue now is whether Donald Trump is going to show up," Podesta said.

Trump and his associates raised questions about the debate schedule last month, arguing that some of the programs will go up against NFL games.

Going forward, the questions are how long can Trump keep his cool and whether he can avoid eruptions on Twitter and inopportune comments that undermine the more level-headed persona he presented in Detroit. After all, predictions that he would become more of a traditional presidential candidate have always foundered in the past.

"The narrative has become: Is Trump really capable? Does he have the temperament to be president?" Patrick Murray, director of the Monmouth University Polling Institute told CNN's Wolf Blitzer. "He has to do something that changes that dynamic."

Seeking to do just that, Trump's speech in Detroit was the most detailed preview yet of how he would run the US economy.

While he maintained his opposition to global trade deals in a way that jars with Republican Party policy, he offered some red meat on tax brackets for GOP lawmakers to embrace.

"My plan will reduce the current number of brackets from seven to three, and dramatically streamline the process. We will work with House Republicans on this plan, using the same brackets they have proposed: 12, 25 and 33%," Trump said.

But Ryan came face-to-face with the complications Trump poses for his party while campaigning in his home state of Wisconsin.

Oscar Bautista, an immigrant from El Salvador, asked whether it was too late to dump Trump and told CNN's Deidre Walsh that he was angry about the GOP nominee's rhetoric toward Latinos.

Ryan, who has attempted to broaden the Republican Party's appeal, said there was nothing he could do.

Trump "won the votes fair and square. ... He got 14 million votes and nobody else got close to that. And so that's just the way the system works," Ryan said.