The House's impeachment inquiry into President Donald Trump has done more than threaten the president's political survival and alter the landscape of his 2020 reelection bid. It could also help to shape some of the most important races in the fight for control of the Senate next year. The vast majority of Republicans, particularly those who face GOP primary challenges, sprang to the president's defense amid the probe. But some senators who are vulnerable next year in blue-leaning states took a more cautious approach as they watch how the investigation into Trump unfolds. At least one Democratic senator — Doug Jones of Alabama — has also tried to walk a fine line between voicing his concerns about Trump's conduct and courting conservative voters while running in a firmly red state. Support for the impeachment inquiry has climbed nationally since Democrats announced it. However, it is less clear now how voters in specific Senate battleground states feel about impeachment. Here is what 10 senators running in races that will help to determine the Senate majority have said about impeachment.

Sen. Doug Jones, D-Ala.

Surrounded by family, freshman Democratic newly sworn-in Senator Doug Jones (D-AL) is mock sworn in by Vice President Mike Pence, with former Vice President Joe Biden, in the Old Senate Chamber on Capitol Hill in Washington DC on Wednesday January, 3 2018. Melina Mara | The Washington Post | Getty Images

Jones, perhaps the most endangered senator in 2020 because he represents deep-red Alabama, has not explicitly backed impeachment. He has called on Republicans and Democrats not to rush to judgment or worry about the political implications as they consider whether Trump abused his office by urging Ukraine's president to investigate his political rival Joe Biden during a July phone call. In a Senate floor speech last week, Jones said the allegations against Trump are "stunning and disturbing." He added that they "go to the heart of national security." But Jones did not join many of his Democratic colleagues in calling for Trump to be removed from office. "Our duties to discharge that office [of senator] are to be fair, to be impartial, to be deliberative, not political," Jones said. "Our duties to this office are to our constituents, to do the very best that we can in making sure that we analyze whatever is in front of us because history will judge us. They will determine whether or not we acted with courage and conviction or whether we just simply tested the political winds, as some people are already doing." Jones' potential opponents next year have jumped to support Trump in a state the president won by nearly 30 percentage points in 2016. Rep. Bradley Byrne, a House Republican running in the GOP Senate primary, tweeted Friday that Democrats "are hell-bent on sabotaging" Trump. Two other candidates, former college football coach Tommy Tuberville and former Alabama Supreme Court Judge Roy Moore, have argued along with Trump that Congress should focus on unsubstantiated allegations of wrongdoing against Biden rather than the president's conduct. Election forecaster ratings: Cook Political Report: Toss-up

Inside Elections: Lean R

Sabato's Crystal Ball: Leans R

Sen. Cory Gardner, R-Colo.

Senator Cory Gardner speaks to supporters during a private campaign rally for Gardner in The Dome at AMG on August 19, 2019 in Greenwood Village Colorado. Helen H. Richardson | The Denver Post via Getty Images | Getty Images

Gardner faces one of the toughest paths to Senate reelection next year in a state that tilts blue. While he has called for transparency about the whistleblower complaint alleging misconduct by Trump and White House officials, he has also criticized Pelosi for starting impeachment proceedings. "I joined my Senate colleagues in unanimously supporting the release of the whistleblower report, and I support the Senate Intelligence Committee's on-going bipartisan review to gather all of the facts," Gardner said in a statement. "Nancy Pelosi's impeachment inquiry to appease the far-left isn't something the majority of Americans support and will sharply divide the country." Two of the leading Democratic candidates aiming to challenge Gardner next year have tried to pressure him over Trump's actions on Ukraine. Former Colorado Gov. John Hickenlooper tweeted last week that Gardner should "speak out demanding a full inquiry with all relevant national security, intelligence and White House advisers testifying in full." Former Colorado House Speaker Andrew Romanoff has gone further. In a tweet Thursday, he said, "This is a president begging to be impeached. Congress should oblige him." Democrats have performed well in Colorado in recent years. Hillary Clinton beat Trump by about 5 percentage points in the state in 2016. Last year, Democratic Gov. Jared Polis won by more than 10 percentage points. Election forecaster ratings: Cook Political Report: Toss-up

Inside Elections: Toss-up

Sabato's Crystal Ball: Toss-up

Sen. Martha McSally, R-Ariz.

Senator Martha McSally, a Republican from Arizona, questions Jerome Powell, chairman of the U.S. Federal Reserve, during a Senate Banking Committee hearing in Washington, D.C., U.S., on Tuesday. Feb. 26, 2019. Andrew Harrer | Bloomberg | Getty Images

McSally has tried not to address Trump's conduct since Pelosi announced the impeachment inquiry. Instead, she has publicly called the proceedings politically dangerous for Democrats and a distraction from issues voters care about. "Literally they are on a path to reelect the president, keep the Senate majority [Republican] and possibly flip the House. It's a total distraction," the senator told Politico. "For the people I represent, this is not what they're talking about." McSally's office did not respond to CNBC's requests to comment on the senator's stance on impeachment. McSally will likely face a tough race in 2020 in Arizona, a state where she lost last year to Democratic Sen. Kyrsten Sinema. She will also have at least one primary challenger in businessman Daniel McCarthy. Democratic former astronaut Mark Kelly will likely face McSally in the general election next year. In an interview with The View last week, he called the allegations against Trump "serious" and said they "should be investigated." He did not answer whether the president's actions met the impeachable standard of high crimes and misdemeanors. Election forecaster ratings: Cook Political Report: Toss-up

Inside Elections: Toss-up

Sabato's Crystal Ball: Toss-up

Sen. Susan Collins, R-Maine

Sen. Susan Collins, R-Maine, testifies during a Senate Energy and Natural Resources Subcommittee on Energy legislative meeting in Dirksen Building on numerous bills on Tuesday, July 09, 2019. Tom Williams | CQ-Roll Call Group | Getty Images

Collins, one of the more centrist Senate Republicans, has not gone into much detail about her views on the impeachment inquiry. After Pelosi announced the inquiry, and following the release of a summary of Trump's phone call with Ukrainian President Volodymyr Zelensky, Collins said she should not comment because of her potential role as a juror in a Senate trial of Trump. "As such, at this point, it is not appropriate for a Senator to comment on the merits of the House inquiry or to prejudge its outcome. Therefore, I will not be commenting on the House proceedings," she said last week. Collins later criticized Trump when he said the whistleblower who highlighted his conduct was "close to a spy." Last week, she said, "whistleblowers have been essential in bringing to the public's attention wrongdoings, fraud, waste, abuse, law breaking, and I very much disagree with the president's mischaracterization." The longtime senator faces reelection next year in a state that leans blue. Sara Gideon, the Democratic Maine House speaker and the leading candidate to challenge Collins, has said she supports starting an impeachment inquiry. "Congress should investigate, and the White House should be forced to comply," she said in a statement last week. "But Congress should be able to do its job of overseeing the President while also doing the jobs they were elected to do — like lowering the cost of prescription drugs and making sure that everyone has access to health care." Election forecaster ratings: Cook Political Report: Toss-up

Inside Elections: Tilt R

Sabato's Crystal Ball: Leans R

Sen. Thom Tillis, R-N.C.

Senator Thom Tillis, a Republican from North Carolina, questions William Barr, U.S. attorney general, not pictured, during a Senate Judiciary Committee hearing in Washington, D.C., U.S., on Wednesday, May 1, 2019. Al Drago | Bloomberg | Getty Images

Tillis has argued Trump's conduct related to Ukraine and the Biden family does not warrant removing the president from office. "There was nothing on its face that I would think would rise to the level of what the House has chosen to do," he said last week, according to the Raleigh, North Carolina-based News & Observer newspaper. Tillis has pushed to show his support for Trump as he faces a primary challenge from two Republicans, businessman Garland Tucker and farmer Sandy Smith. Meanwhile, he will have to hold off one of several Democrats aiming to unseat him next year in a state where Democratic Gov. Roy Cooper won in 2016 even with Trump at the top of the ticket. The campaign of one of the leading candidates in the Democratic primary to challenge Tillis, former state Sen. Cal Cunningham, did not immediately respond to a request to comment on the impeachment inquiry. Another potential Tillis opponent, state Sen. Erica Smith, has said she supports the House's decision to start impeachment proceedings. Election forecaster ratings: Cook Political Report: Lean R

Inside Elections: Tilt R

Sabato's Crystal Ball: Toss-up

Sen. Gary Peters, D-Mich.

Sen. Gary Peters, D-Mich. Bill Clark | CQ-Roll Call Group | Getty Images

Peters, running for reelection in a state Trump narrowly won in 2016, has taken a deliberate approach to the House investigation. In a statement last week, he said Trump's words and the whistleblower complaint "raise serious questions that require answers." "This should not be a partisan process. We must focus on finding the facts, and it is imperative that Congress continue working to fulfill its constitutional oversight responsibility," he said. Peters added that if impeachment came before the Senate he would reach his decision "based on a careful and thorough evaluation of the facts." The senator is the ranking Democrat on the Homeland Security and Governmental Affairs Committee, and wrote a letter to the Office of Management and Budget last week asking for more information on the administration's decision to withhold military aid to Ukraine earlier this year. John James, an Army veteran and businessman who lost to Democratic Sen. Debbie Stabenow last year, will likely face Peters in 2020. James' campaign did not immediately respond to a request to comment on where the GOP candidate stands on impeachment. Election forecaster ratings: Cook Political Report: Likely D

Inside Elections: Likely D

Sabato's Crystal Ball: Leans D

Sen. Joni Ernst, R-Iowa

Sen. Joni Ernst, R-Iowa, talks with fairgoers as she works at the Pork Tent at the Iowa State Fair on Saturday August 17, 2019. Caroline Brehman | CQ-Roll Call Group | Getty Images

Ernst, like many of her colleagues, said she saw "nothing there" after the Trump administration released its summary of the call with Zelensky last week. She argued the inquiry would distract from legislating. Even so, she has distanced herself from Trump on one key point. The president has repeatedly criticized the intelligence community whistleblower and pushed to identify the person. Ernst joined her Iowa colleague, GOP Sen. Chuck Grassley, in supporting the whistleblower's right to anonymity. "Whistleblowers should be protected. I stand with Chuck Grassley on this," she said at a town hall Thursday. "We have laws in place. Again, laws need to be enforced." Trump won Iowa by nearly 10 percentage points, but his approval rating there has fallen since he took office. Trump's ongoing trade war with China has hung over the state, which relies on agricultural exports. Some of Ernst's potential Democratic opponents such as businessman Eddie Mauro and lawyer Kimberly Graham have started to pressure the senator over impeachment. Eddie Mauro tweet Kimberly Graham tweet Theresa Greenfield, a leading candidate in the primary, said "Congress needs to do its job in a bipartisan way to uncover the facts on the disturbing allegations" against Trump. She said "it's disappointing" to see Ernst "dismissing this alarming information." Election forecaster ratings: Cook Political Report: Likely R

Inside Elections: Likely R

Sabato's Crystal Ball: Leans R

Sen. David Perdue, R-Ga.

Sen. David Perdue (R-GA) questions U.S. Secretary of State Mike Pompeo during a Senate Caucus on International Narcotics Control hearing on June 11, 2019 in Washington, DC. Zach Gibson | Getty Images

Perdue, a vocal Trump ally, has downplayed the president's efforts to get Zelensky to investigate Biden. "There is absolutely nothing in this phone call that rises to the level of that (impeachment)," he said last week, according to The Associated Press. Democrats have high hopes for the red-leaning state of Georgia after a closer-than-expected gubernatorial race there last year. Challengers have piled into the primary to take on Perdue, and the senator's potential opponents have been vocal about impeachment. Businesswoman and 2018 lieutenant governor candidate Sarah Riggs Amico tweeted last week that she supports the inquiry "given the seriousness of the allegations made against the president." Former Columbus, Georgia, Mayor Teresa Tomlinson went further: she tweeted Monday that "we cheated ourselves" by not starting the impeachment proceedings sooner. Jon Ossoff, a Senate candidate who ran in a highly publicized House special election last year, also tweeted last week that Trump "should be impeached" if the allegations against him are true. Two Georgia Senate seats will be up for grabs next year after GOP Sen. Johnny Isakson's retirement at the end of the year. It is unclear now who will get appointed to Isakson's seat. Election forecaster ratings: Cook Political Report: Likely R

Inside Elections: Lean R

Sabato's Crystal Ball: Leans R

Sen. Jeanne Shaheen, D-N.H.

Senator Jeanne Shaheen, D-N.H., speaks at U.S. Capitol press conference about seniors facing higher prescription drug prices if Obamacare is overturned. Michael Brochstein | SOPA Images | LightRocket | Getty Images

Shaheen is one of the only Senate Democrats running for reelection in a state where Trump was competitive in 2016: he lost New Hampshire by fewer than 3,000 votes. She has said she wants to hear from witnesses about Trump's conduct but noted that she would like to see a probe done "very expeditiously." "I think it raises very serious concerns about what the President has done and about the potential for a cover-up and that's why we need the facts to come out and the American people need to see what happened," Shaheen said last week, according to the New Hampshire Union Leader newspaper. "We need to hear from people who know what happened inside the White House." At least one of her potential opponents next year has downplayed Trump's actions. Bill O'Brien, a GOP Senate candidate and former speaker of the New Hampshire House, said he did not see direct evidence in the July call of Trump withholding military aid in exchange for a Biden investigation. "There has to be a quid pro quo, and there's none there," he said last week, according to the Union Leader. Election forecaster ratings: Cook Political Report: Solid D

Inside Elections: Likely D

Sabato's Crystal Ball: Leans D

Sen. John Cornyn, R-Texas

John Cornyn, a Republican from Texas and chairman of the Senate Judiciary Border Security and Immigration Subcommittee, makes an opening statement during a hearing in Washington, D.C., U.S., on Wednesday, May 8, 2019. Andrew Harrer | Bloomberg | Getty Images