The elected Republican lawmakers who have jumped ship from presidential nominee Donald Trump’s campaign span east and west, north and south. Some are conservative, and others more moderate. They are men and women.

And to a person, they say Mr. Trump isn’t a serious enough candidate, either in personality or policy, for them to back him.

Sen. Susan M. Collins, a Maine Republican who tilts toward the moderate side of her party, became the latest to sign on with the anti-Trump forces, saying this week that she is looking for a candidate other than Mr. Trump or Democratic nominee Hillary Clinton.

“Most of these members either think Trump is too immoderate in tone and temperament for their tastes or for their districts/states, or that Trump isn’t conservative enough on the issues they care about,” said Kyle Kondik, of the University of Virginia’s Center for Politics.

Mr. Trump dismisses the defections, saying those who refuse to back him aren’t the types of supporters he wants anyway. “I am running against the Washington insiders, just like I did in the Republican Primaries,” he said on Twitter Tuesday. “These are the people that have made U.S. a mess!”

But the defectors continue to derail the billionaire businessman’s efforts to right a campaign beset by gaffes, with Ms. Collins’ decision spurring another round of negative coverage for Mr. Trump, who has lost ground to Mrs. Clinton in polls.

“I have always supported my party’s nominee. That’s what made this decision so difficult,” Ms. Collins told CNN on Tuesday. “But in the end, I just cannot support Donald Trump. I do not believe that he is the president that we need at this time in our country’s history, and I believe that in many ways, he’s antithetical to the values of the Republican Party.”

Sens. Ben Sasse of Nebraska, Lindsey Graham of South Carolina and Jeff Flake of Arizona are other Republicans who say they cannot back Mr. Trump.

Other Senate Republicans have come down on the side of their nominee — with varying degrees of enthusiasm.

Sen. Jeff Sessions, an Alabama Republican, has been one of Mr. Trump’s most prominent surrogates, and Senate Majority Leader Mitch McConnell of Kentucky endorsed Mr. Trump and defended him at the Republican National Convention last month.

But senators seeking re-election in swing states have struggled to calibrate their responses.

Sen. Mark Kirk of Illinois was for Mr. Trump before he was against him. Sens. Kelly Ayotte of New Hampshire, John McCain of Arizona and Patrick J. Toomey of Pennsylvania have embraced Mr. Trump in the most lukewarm of fashions while trying to keep him at more than an arm’s length

“Right now, I think Toomey has made it very clear that Donald Trump was not his first choice for president, he was not his second choice, but he is on the ticket with him and that is where it is,” said Charlie Gerow, a Pennsylvania-based Republican Party strategist. “I believe Toomey has played it just right.”

Sen. Ted Cruz of Texas and Sen. Mike Lee of Utah are also holdouts, saying Mr. Trump has not proved he is a conservative.

In the House chamber, Rep. lleana Ros-Lehtinen of Florida, a supporter of legalizing illegal immigrants, and Rep. Carlos Curbelo, who is locked in a tight re-election fight in Florida’s 26th Congressional District and has teased the idea of voting for Mrs. Clinton, have come out against Mr. Trump.

So have Rep. Charles W. Dent of Pennsylvania, a star among libertarians, and Adam Kinzinger and Robert J. Dold, both of Illinois.

Rep. Justin Amash of Michigan, a star among libertarians, has refused to endorse Mr. Trump.

A trio of retiring House members — Reid J. Ribble of Wisconsin, Richard L. Hanna of New York and Scott E. Riggell of Virginia — also have ditched the party’s nominee. For now, Mr. Hanna is the only Republican member of Congress to say he will cross the aisle to vote for Mrs. Clinton. Mr. Riggell said this week that he has moved into Libertarian presidential nominee Gary Johnson’s camp.

“The nominee for our party, the Republican nominee, I am just convinced is just so lacking in judgment and temperament and in character that I think he really represents a true risk to our country,” Mr. Riggell said Tuesday on MSNBC’s “Morning Joe.”

Warming up a crowd for Mr. Trump on Tuesday in North Carolina, former New York Mayor Rudolph W. Giuliani told voters that the naysayers are “why you should vote for him. Trump gets the establishment nervous.”

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