Here's a semi-comprehensive list we'll be updating of the many notable Republicans who have said they won't support GOP presidential nominee Donald Trump, why they're opposing him and who they'll back instead:

Senators

Susan Collins: The Maine senator wrote an op-ed in The Washington Post on August 8 stating that although she is a lifelong Republican, she will not support the party's nominee -- a decision she says she does not take "lightly." "Donald Trump does not reflect historical Republican values nor the inclusive approach to governing that is critical to healing the divisions in our country," Collins wrote.

Supporting: Not Trump or Clinton; said she would consider voting for the Libertarian ticket, may write in a name.

JUST WATCHED Sen. Susan Collins: Trump is antithetical to GOP values Replay More Videos ... MUST WATCH Sen. Susan Collins: Trump is antithetical to GOP values 01:48

Mark Kirk: The Illinois senator said June 8 that he "cannot and will not support" Trump -- who he said "does not have the temperament" to be commander in chief. Kirk faces a tough challenge in November from Illinois Democratic Rep. Tammy Duckworth.

Supporting: Kirk told CNN that he will write in former CIA director David Petraeus.

Sen. Mark Kirk (R-IL) questions a group of nuclear experts at a public forum on the safety of Illinois nuclear power plants on March 25, 2011 in Chicago, Illinois.

Ben Sasse: The freshman senator from Nebraska is the first in the Senate to consistently and vocally oppose Trump, who he has said "does not even know what the Constitution is about." He urged voters to vote their conscience in a post on Medium July 11, and has said he will support neither Trump nor Clinton.

Supporting: Sasse has suggested he may look for a third candidate.

Sen. Ben Sasse (R-NE) (3L) arrives at the Capitol for a cloture vote on the Keystone XL Pipeline January 26, 2015 on Capitol Hill in Washington, DC.

Lindsey Graham: The South Carolina senator told CNN May 6 that he won't vote for Trump or Clinton. Saying he thinks the Republican Party has been "conned," Graham added that he doesn't think Trump is "a reliable Republican conservative."

Supporting: Hasn't said publicly.

Sen. Lindsey Graham (R-SC) speaks at the Growth and Opportunity Party, at the Iowa State Fair October 31, 2015 in Des Moines, Iowa.

Ted Cruz: The Texas senator and former Trump rival hasn't explicitly said he won't vote for Trump, but he pointedly refused to endorse the nominee during a prime-time speech at the Republican National Convention. He defended his decision the next day, saying he's not "in the habit" of backing politicians who attack his family.

Supporting: Cruz hasn't said who he supports, but he urged voters at the RNC to follow their "conscience."

JUST WATCHED Ted Cruz to Republicans: Vote your conscience Replay More Videos ... MUST WATCH Ted Cruz to Republicans: Vote your conscience 00:48

Jeff Flake: The Arizona senator doesn't support the nominee, but he said on CBS' "Face the Nation" August 7 that he still hopes to. "It's becoming increasingly difficult to see that he's going to make the changes that he needs," Flake added.

Supporting: Not Clinton; still holding out hope for a big change from Trump

Sen. Jeff Flake (R-AZ) walks to a Senate joint caucus meeting, on Capitol Hill, July 15, 2013 in Washington, DC.

Dean Heller: The Nevada senator Dean Heller told Politico June 30 that he Trump "did a lot of damage" and that he thinks it'll be difficult for the Republican nominee to "recover from his previous comments. "I'll give him a chance, but at this point, I have no intentions of voting for him," Heller said.

Supporting: Hasn't said publicly.

Sen. Dean Heller (R-NV) speaks during a rally for Republican presidential candidate Sen. Marco Rubio (R-FL) at the Texas Station Gambling Hall & Hotel on February 21, 2016 in North Las Vegas, Nevada.

Representatives

Scott Rigell: The House member, who announced he won't run for re-election in his Virginia district, said August 6 that he will vote for Libertarian Gary Johnson for president over his party's nominee.

Supporting: Johnson.

Rep. Scott Rigell (R-VA) talks on his phone as he arrives at a House Republican Conference meeting September 28, 2013 on Capitol Hill in Washington, DC.

Richard Hanna: The New York lawmaker was the first Republican member of Congress to openly announce he will vote for Hillary Clinton in November. In an op-ed for Syracuse.com August 2, Hanna -- who is retiring after this term -- wrote that Trump is "deeply flawed in endless ways" and "unrepentant in all things" then declared: "While I disagree with her on many issues, I will vote for Mrs. Clinton."

Supporting: Clinton.

Rep. Richard Hanna (R-NY) (R) and representatives of women's groups hold a rally to mark the 40th anniversary of congressional passage of the Equal Rights Amendment (ERA) outside the U.S. Capitol March 22, 2012 in Washington, D.C.

Adam Kinzinger: The Illinois lawmaker told CNN's Wolf Blitzer in August that he doesn't see how he can support Trump following his comments about NATO and the Khan family. "I'm an American before I'm a Republican," he told Blitzer. But Kinzinger said he would not support Clinton.

Supporting: Most likely writing in a candidate.

JUST WATCHED Republican doesn't think he can support Donald Trump Replay More Videos ... MUST WATCH Republican doesn't think he can support Donald Trump 01:26

Ileana Ros-Lehtinen: The senior member of Florida's congressional delegation told the Miami Herald May 6 that she won't back either Trump or Clinton. Ros-Lehtinen, who was born in Cuba and emigrated to the U.S., said she will "work with whomever is chosen by the American people to serve as president," but noted, "In this election, I do not support either Donald Trump or Hillary Clinton."

Supporting: Will write in Jeb Bush

House Foreign Affairs Committee members Rep. Chris Smith (R-NJ) (L) and Rep. Ileana Ros-Lehtinen (R-FL) talk before a hearing about Cuba policy in the Rayburn House Office Building on Capitol Hill February 4, 2015 in Washington, DC.

Charlie Dent: The Pennsylvania House member said on MSNBC on August 2 that he will not support Trump or Clinton and has "no plans" to vote for Trump. "I think some of us have to stand up once in a while and just have to say how we feel about this," Dent said.

Supporting: Hasn't said publicly.

Congressman Charlie Dent, Representative for Pennsylvania's 15th congressional district, attends the 2015 Global Citizen Festival to end extreme poverty by 2030 in Central Park on September 26, 2015 in New York City.

Mike Coffman: The Colorado representative aired an ad August 4 publicly voicing his unhappiness with his party's nominee -- speaking directly into the camera, he says: "People ask me, 'What do you think about (Donald) Trump?' Honestly, I don't care for him much." The congressman, a Marine who served in the Army reserves, said he wouldn't rule out voting for Trump -- though he said it's hard to imagine the campaign changing enough that he would lend his support.

Supporting: Not ruling out Trump, considering Johnson.

Chairman Mike Coffman (R-CO) speaks during a House Veterans Affairs Subcommittee hearing on April 13, 2014 in Washington, DC.

Bob Dold: One of the first Republicans to oppose Trump, the Illinois representative told WLS May 6: "I will not support Donald Trump," referring to Trump's comments about women, Muslims, Latinos and POWs. "We're looking for a uniter, not a divider," he said.

Supporting: Will write in a candidate.

Rep. Mike Thompson (D-CA), Rep. Bob Dold (R-IL), Rep. Elizabeth Esty (D-CT) and Rep. Kathleen Rice (D-NY) hold a news conference about new legislation to enforce background checks for gun purchases in the Canon House Office Building on Capitol Hill March 4, 2015 in Washington, D.C.

Governors

John Kasich: The Ohio governor and Trump's former rival has been one of the most prominent figures to withhold his support from Trump -- although he hasn't explicitly said he won't vote for the nominee. He told CNN August 5 that he wishes he could be "fully enthusiastic," but said, "I don't know what's going to happen at the end."

Supporting: Says he hasn't decided yet.

JUST WATCHED Kasich: I might not vote GOP for president Replay More Videos ... MUST WATCH Kasich: I might not vote GOP for president 05:58

Larry Hogan: The governor of Maryland said June 15 that he "doesn't plan" to vote for Trump.

Supporting: Hogan says he's "not sure" who he'll vote for.

Gov. Larry Hogan (R-MD) (R), listens to questions during a meeting with members of the National Governors Association, in the State Dining Room at the White House, February 22, 2016 in Washington, DC.

Charlie Baker: The Massachusetts governor said he likely won't support Trump or Clinton. "The things he said about women and Muslims and religious freedom, I just can't support," Baker said. "At the same time, I do believe Secretary Clinton has a huge believability problem."

Supporting: Hasn't said publicly

Gov. Charlie Baker of Massachusetts and wife Lauren Baker march in the annual South Boston St. Patrick's Parade passes on March 20, 2016.

Party elders

Jeb Bush: The ex-Florida governor and Trump's former rival announced on Facebook May 6 that he would not vote for Trump or Clinton -- stating the Republican nominee has not demonstrated the "temperament or strength of character" to be president.

Supporting: Not Trump or Clinton, but "will support principled conservatives at the state and federal levels."

Jeb Bush speaks to an audience of voters on February 18, 2016 in Columbia, South Carolina.

Barbara Bush: The former first lady told CNN's Jamie Gangel February 5 that Trump has said "terrible things about women, terrible things about the military." "I don't even think about him," she said. "I'm sick of him."

Supporting: Hasn't said publicly.

President George H.W. Bush and his wife, Mrs. Barbara Bush attend the HBO Documentary special screening of "41" on June 12, 2012 in Kennebunkport, Maine.

Mitt Romney: The 2012 Republican nominee has long openly opposed Trump's candidacy, telling CNN June 10 that his election could legitimize racism and misogyny in America.

Supporting: Has said he wouldn't rule out voting for the Libertarian ticket.

JUST WATCHED Mitt Romney: 'This is breaking my heart' Replay More Videos ... MUST WATCH Mitt Romney: 'This is breaking my heart' 00:45

Larry Pressler: The former three-term senator from South Dakota endorsed Clinton June 12 following the Orlando terror attack, stating that she "would be able to handle such explosive situations which are terrorist inspired much better than Donald Trump."

Supporting: Clinton.

Former senator Larry Pressler attends the Jericho Project Gala "Celebrate Them Home" at Pier Sixty at Chelsea Piers on April 29, 2010 in New York City.

Vin Weber: The former ally of Newt Gingrich and a representative from Minnesota for 12 years told CNBC August 3 that he rejects Trump's candidacy -- calling his nomination "a mistake of historic proportions." Weber said he hasn't decided if he'll vote for Clinton.

Supporting: Hasn't decided.

Mexican President Vicente Fox (L) accepts the National Endowment For Democracy's (NED) 2001 Award on Capitol Hill in Washington from Vin Weber of the NED.

Gordon Humphrey: The former New Hampshire senator told NBC News August 4 that he won't support Trump, and would vote for Clinton if the contest in his home state, New Hampshire, is close. "I am ever more confirmed in my belief that Trump is a sociopath, without a conscience or feelings of guilt, shame or remorse," Humphrey wrote.

Supporting: Clinton -- but only if the race in New Hampshire is close.

Gordon Humphrey, speaks to the media on the first day of the Republican National Convention on July 18, 2016 at the Quicken Loans Arena in Cleveland, Ohio.

Chris Shays: Former congressman Chris Shays of Connecticut wrote in an Former congressman Chris Shays of Connecticut wrote in an op-ed on CNN that, "I think many Republicans know Donald Trump could cause great damage to our country and the world at large, and still plan to vote for him. But not me. He represents practically everything I was taught not to be, and everything my wife and I taught our daughter not to be."

Supporting: Clinton

William Milliken: The former Michigan governor said in a statement August 8 that he won't support Trump, and will cast his vote for Clinton, according to the Detroit Free Press.

Supporting: Clinton

Party insiders

Sally Bradshaw: Jeb Bush's former top adviser told CNN August 2 that she has left the Republican Party, and that if the race is close in her home state of Florida, she will vote for Clinton. Bradshaw said the Republican Party is "at a crossroads and have nominated a total narcissist -- a misogynist -- a bigot."

Supporting: Clinton -- but only if the race in Florida is close

Florida State Board of Education member Sally Bradshaw, gestures as she interviews finalists for the Commissioner of Education Tuesday, Dec. 11, 2012, in Tampa, Fla.

Maria Comella: Chris Christie's longtime aide told CNN August 2 she will vote for Clinton -- calling Trump "a demagogue" who preys "on people's anxieties with loose information and salacious rhetoric, drumming up fear and hatred of the 'other.'"

Supporting: Clinton

Maria Comella helped elevate Gov. Chris Christie's national image.

Meg Whitman: The president and CEO of Hewlett-Packard and a major GOP donor said August 2 that she will support Clinton over Trump -- stating that Trump's "demagoguery has undermined the fabric of our national character." The former California gubernatorial candidate said she would contribute to Clinton's campaign and its allied super PACs.

Supporting: Clinton

Newly formed Hewlett-Packard Enterprise Chief Executive Officer (CEO) Meg Whitman speaks during a press conference in New York on November 2, 2015.

Hank Paulson: The Republican Treasury secretary endorsed Clinton June 24, writing in an op-ed in The Washington Post that "a Trump presidency is unthinkable." "To my Republican friends: I know I'm not alone," Paulson wrote.

Supporting: Clinton

Former U.S. Treasury Secretary Henry Paulson testifies during a hearing before the Financial Crisis Inquiry Commission May 6, 2010 on Capitol Hill in Washington, DC.

Marc Racicot: The former governor of Montana and former chairman of the Republican National Committee wrote in an op-ed in The Washington Post that he "cannot endorse or support" Trump.

Supporting: "Honestly, I do not know what I'm going to do," he told Bloomberg Politics.

Bush-Cheney '04 Campaign Chairman Marc Racicot speaks during the opening session of the 2004 RNC August 30, 2004 at Madison Square Garden in New York City.

Mark Salter: The former top adviser to Sen. John McCain tweeted his support for Clinton on May 3, writing; "the GOP is going to nominate for President a guy who reads the National Enquirer and thinks it's on the level. I'm with her."

Supporting: Clinton

Mark Salter speaks to the travelling press after McCain announced he was suspending his campaign September 24, 2008 at a hotel in New York City.

Lezlee Westine: The former aide to President George Bush announced August 8 she will support Clinton -- writing that the country "faces a unique set of challenges that require steady and experienced leadership."

Supporting: Clinton

Christine Todd Whitman: The former governor of New Jersey said March 1 she would "probably" vote for Clinton over Trump. "While I certainly don't want four more years of another Clinton administration or more years of the Obama administration, I would take that over the kind of damage I think Donald Trump could do to this country, to its reputation, to the people of this country," Whitman said.

Supporting: "Probably" Clinton

Christine Todd Whitman attends Making Strides: Advancing Women's Leadership - Opening Reception at Vizcaya Museum & Gardens on November 17, 2015 in Miami, Florida.

George Will: The conservative commentator and columnist said June 26 that he left the Republican Party because of Trump -- "I decided that in fact this was not my party anymore," he said.

Supporting: Hasn't said publicly

Syndicated Columnist George Will speaks during the 18th Annual Borton, Petrini & Conron, LLP's Bakersfield Business Conference on October 12, 2002 in Bakersfield, California.

Paul Singer: The Republican megadonor said in May he will not support Trump or Clinton.

Supporting: Hasn't said publicly

Founder and President, Elliot Management Corporation Paul Singer speaks onstage during The New York Times DealBook Conference at One World Trade Center on December 11, 2014 in New York City.

Charles and David Koch: The Republican donors have withheld their support from Trump, and Charles Koch suggested in April it was "possible" Clinton would be a better president.

Supporting: Haven't said publicly

National security and foreign policy leaders

National security experts: A group of 50 leaders in the Republican foreign policy and national security community wrote an open letter condemning Trump and pledging to oppose his presidential candidacy Monday. "We are unable to support a Party ticket with Mr. Trump at its head," they wrote in the letter.

The authors of the letter included former director of the National Security Agency Michael Hayden; Michael Chertoff, the director of Homeland Security under George W. Bush; John Negroponte, the former Director of National Intelligence; Robert Kagan, a leader of the neoconservative movement and a member of the Council on Foreign Relations; Eric Edelman, former Under Secretary of Defense Policy during the first Bush administration and U.S. Ambassador to Turkey; and dozens of other former Republican presidential administration officials and conservative think tank members.

Richard Armitage: The deputy secretary of state under George W. Bush told Politico June 16 he will vote for Clinton over Trump. A retired Navy officer who also served as an assistant secretary of defense under Ronald Reagan, Armitage said Trump "doesn't appear to be a Republican, he doesn't appear to want to learn about issues."

Supporting: Clinton

Richard Armitage, former US Deputy Secretary of State, listens as Admiral Katsutoshi Kawano, Japan's chief of staff of the Joint Staff Council and Self-Defense Forces, speak at the Center for Strategic and International Studies in Washington, DC on July 16, 2015.

Brent Scowcroft: The National Security Adviser to Presidents George H. W. Bush and Gerald Ford, who worked in the White House of Presidents Richard Nixon and George W. Bush, endorsed Clinton June 23. He said the former secretary of state "brings truly unique experience and perspective to the White House."

Supporting: Clinton

Former National Security Advisor Brent Scowcroft testifies during a Senate Armed Services Committee hearing on Capitol Hill, January 21, 2015 in Washington, DC.

Names released by the Clinton campaign

The Clinton campaign released a list of Republicans, former Republicans and Independents who it said were supporting Clinton. It included many of the names on this list, but also included former New York Mayor Michael Bloomberg, now an independent; Rep. Connie Morrella of Maryland; Arne Carlson, the former Gov. of Minnesota; and former GM CEO Dan Akerson.

Supporting: Clinton

Correction: An earlier version of this story misidentified the state Ben Sasse represents.