WILMINGTON, N.C. — Donald Trump is seeking to quell concerns he lacks the discipline or policy know-how to make a competent president, even as the list of fellow Republicans deeming him unfit for the Oval Office grows.

Maine Sen. Susan Collins, a moderate long wary of Trump, became the latest Republican to announce her intent not to vote for her party’s nominee. Days after rebuking Trump for insinuating Somali refugees in Maine were dangerous, Collins said late Monday she’d thought “long and hard” about whether she was obligated to support the GOP nominee and decided she could not.

“With the passage of time, I have become increasingly dismayed by his constant stream of cruel comments and his inability to admit error or apologize,” Collins wrote in a Washington Post op-ed.

Collins wrote that she supports neither party’s nominee, though previously she’s said she’s open to voting for Hillary Clinton.

The defection from a senator added to other GOP voices insisting they can’t back Trump. Some 50 Republican former national security officials signed an open letter calling Trump the most reckless candidate in history, prompting a counterattack from Trump, who said the signers share blame with Clinton for making the world “a mess” and fueling the Islamic State group’s formation.

Fresh polls appear to show Clinton widening her lead. But Trump suggested Tuesday there would be no dramatic change of strategy to regain control of the race.

“I think it’s just, you know, steadiness,” Trump told Fox Business. “And it’s just doing what I’m doing.”

A day earlier, Trump in a major policy speech at the Detroit Economic Club unveiled a revamped economic plan centered on far-reaching tax cuts.

Clinton quickly dismissed Trump’s proposal, which would reduce to three the number of income tax brackets and cut corporate taxes to 15 percent. She accused Trump of offering “super big tax breaks” to huge companies and rich people and disputed his claim that she wanted the middle class to pay more.

“I have said throughout this campaign I am not going to raise the taxes on the middle class, but with your help we are going to raise it on the wealthy,” Clinton said at a Monday rally in Florida.

Late Monday, Clinton’s campaign chairman announced she would take part in all three debates that the Commission on Presidential Debates is organizing. Trump has said he wants to debate Clinton but has complained that two of the debates are scheduled during NFL football games, claiming Democrats “rigged” the schedule.

Clinton, working to shore up a path to victory in the Electoral College, was focusing intently on Florida. She planned to tour a Miami health clinic today to discuss the Zika virus before holding fundraisers in the evening.

Trump, too, had his eye on the most competitive states. A day after campaigning in Michigan, the real estate mogul planned a pair of rallies in North Carolina.