A trio of Senate Republicans eyeing the White House in 2016 on Wednesday went on record as saying climate change is not a hoax, but split on the question of whether human activity is causing it.

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Sens. Marco RubioMarco Antonio RubioRubio responds to journalist who called it 'strange' to see him at Trump rally Rubio responds to journalist who called it 'strange' to see him at Trump rally Overnight Defense: US to send 1K more troops to Mideast amid Iran tensions | Iran threatens to break limit on uranium production in 10 days | US accuses Iran of 'nuclear blackmail' | Details on key defense bill amendments MORE (R-Fla.), Ted CruzRafael (Ted) Edward CruzGOP senator introduces bill to hold online platforms liable for political bias GOP lawmaker delays House for second week GOP lawmaker delays House for second week MORE (R-Texas) and Rand PaulRandal (Rand) Howard PaulOvernight Defense: Shanahan exit shocks Washington | Pentagon left rudderless | Lawmakers want answers on Mideast troop deployment | Senate could vote on Saudi arms deal this week | Pompeo says Trump doesn't want war with Iran Senators demand Trump explain decision to deploy troops amid Iran tensions Senators demand Trump explain decision to deploy troops amid Iran tensions MORE (R-Ky.) all voted for an amendment to the Keystone XL pipeline bill that said climate change is real. In turn, they all voted against an amendment from Sen. Brian Schatz (D-Hawaii) that said human activity "significantly" contributes to it.

In a third and final amendment vote, Paul backed a provision that said humans contribute to the changing climate. Rubio and Cruz voted no.

All three senators are all weighing presidential bids and eying a tough primary fight in what's likely to be a crowded GOP field.

The votes could provide fodder for Democratic attacks if any of them win the GOP nomination, or against Rubio if he decides to run for reelection instead in 2016. Rubio has previously expressed skepticism towards the idea that man-caused climate change is settled science.

Paul's vote on human-caused climate change could wind up being a liability with GOP primary voters, but could help burnish his reputation as a Republican who can appeal to Democrats.

There were some fissures in the GOP, however, in a vote designed by Senate Democrats to put their opponents on the spot. Sens. Mark KirkMark Steven KirkEx-GOP Sen. Kirk registers to lobby The global reality behind 'local' problems Dems vow swift action on gun reform next year MORE (R-Ill.), Kelly AyotteKelly Ann AyotteSinema, Gallagher fastest lawmakers in charity race New Hampshire senator to ask 2020 Dems to back repeal of state residency law Schultz recruiting GOP insiders ahead of possible 2020 bid MORE (R-N.H.), Lindsey GrahamLindsey Olin GrahamOvernight Defense: Shanahan exit shocks Washington | Pentagon left rudderless | Lawmakers want answers on Mideast troop deployment | Senate could vote on Saudi arms deal this week | Pompeo says Trump doesn't want war with Iran Shanahan drama shocks Capitol Hill, leaving Pentagon rudderless Shanahan drama shocks Capitol Hill, leaving Pentagon rudderless MORE (R-S.C.), Lamar AlexanderAndrew (Lamar) Lamar AlexanderOvernight Health Care: Poll finds most Americans misunderstand scope of 'Medicare for All' | Planned Parenthood chief readies for 2020 | Drugmakers' lawsuit ramps up fight with Trump Overnight Health Care: Poll finds most Americans misunderstand scope of 'Medicare for All' | Planned Parenthood chief readies for 2020 | Drugmakers' lawsuit ramps up fight with Trump Trump's health care focus puts GOP on edge MORE (R-Tenn.) and Susan CollinsSusan Margaret CollinsOvernight Defense: US to send 1K more troops to Mideast amid Iran tensions | Iran threatens to break limit on uranium production in 10 days | US accuses Iran of 'nuclear blackmail' | Details on key defense bill amendments Overnight Defense: US to send 1K more troops to Mideast amid Iran tensions | Iran threatens to break limit on uranium production in 10 days | US accuses Iran of 'nuclear blackmail' | Details on key defense bill amendments Senators revive effort to create McCain human rights commission MORE (R-Maine) split with their party to vote for the amendment.

Kirk and Ayotte may face tough reelection campaigns next year. Kirk has voted in the past in favor of cap and trade legislation, and has a fairly centrist voting record on environmental issues. Alexander, Graham and Collins recently won reelection and have signaled they're open to working on climate change legislation in the past. Graham has been talking up a potential presidential run, though few think he'll take the plunge.

Other Republicans facing potentially tough reelection campaigns, including Sens. Ron JohnsonRonald (Ron) Harold JohnsonHillicon Valley: Lawmakers angered over Border Patrol breach | Senate Dems press FBI over Russian hacking response | Emails reportedly show Zuckerberg knew of Facebook's privacy issues | FCC looks to improve broadband mapping Hillicon Valley: Lawmakers angered over Border Patrol breach | Senate Dems press FBI over Russian hacking response | Emails reportedly show Zuckerberg knew of Facebook's privacy issues | FCC looks to improve broadband mapping Lawmakers demand answers on Border Patrol data breach MORE (R-Wis.), Rob PortmanRobert (Rob) Jones PortmanHouse passes bill to establish DHS cyber 'first responder' teams House passes bill to establish DHS cyber 'first responder' teams Democrats needle GOP on standing up to Trump MORE (R-Ohio) and Richard BurrRichard Mauze BurrDemocrats detail new strategy to pressure McConnell on election security bills Democrats detail new strategy to pressure McConnell on election security bills Ex-state senator in North Carolina enters race against Tillis MORE (R-N.C.), all voted against the amendment.

Sen. Lisa MurkowskiLisa Ann MurkowskiHillicon Valley: Hacker group targeted electric grid | House Democrats press CBP over facial recognition program | Senators offer bill to protect health data | Groups file FCC complaint over carriers' use of location data Hillicon Valley: Hacker group targeted electric grid | House Democrats press CBP over facial recognition program | Senators offer bill to protect health data | Groups file FCC complaint over carriers' use of location data Klobuchar, Murkowski introduce legislation to protect consumer health data MORE (R-Alaska), who in the past has expressed openness to working on climate change legislation, did as well. Murkowski may face a primary challenge in 2014.

The amendment failed by a 50-49 count, with a 60 vote threshold.

— This story was updated at 7:14 p.m. with details on the final amendment vote.