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 RubioWatch live: Day 2 at CPAC DeSantis derides 'failed Republican establishment' at CPAC The Hill's 12:30 Report - Presented by Facebook - Divided House on full display MORE (R-Fla.), Ted CruzRafael (Ted) Edward CruzNoem touts South Dakota coronavirus response, knocks lockdowns in CPAC speech Sunday shows preview: 2024 hopefuls gather at CPAC; House passes coronavirus relief; vaccine effort continues Texas attorney general hits links with Trump before CPAC appearance MORE (R-Texas) and Rand PaulRandal (Rand) Howard PaulOvernight Health Care: 50 million coronavirus vaccines given | Pfizer news | Biden health nominees Rand Paul criticized for questioning of transgender health nominee Haley isolated after Trump fallout 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 KirkSenate majority battle snags Biden Cabinet hopefuls The Hill's Morning Report - Sponsored by Facebook - Senate makes SCOTUS nominee Barrett a proxy for divisive 2020 Senate Republicans scramble to put Trump at arm's length MORE (R-Ill.), Kelly AyotteKelly Ann AyotteOvernight Defense: NATO expanding troops in Iraq Overnight Defense: New START extended for five years | Austin orders 'stand down' to tackle extremism | Panel recommends Biden delay Afghanistan withdrawal Study group recommends Biden delay Afghanistan withdrawal MORE (R-N.H.), Lindsey GrahamLindsey Olin GrahamOvernight Defense: Biden sends message with Syria airstrike | US intel points to Saudi crown prince in Khashoggi killing | Pentagon launches civilian-led sexual assault commission Graham: Trump will 'be helpful' to all Senate GOP incumbents John Boehner tells Cruz to 'go f--- yourself' in unscripted audiobook asides: report MORE (R-S.C.), Lamar AlexanderLamar AlexanderCongress addressed surprise medical bills, but the issue is not resolved Trump renominates Judy Shelton in last-ditch bid to reshape Fed Senate swears-in six new lawmakers as 117th Congress convenes MORE (R-Tenn.) and Susan CollinsSusan Margaret CollinsCollins urges Biden to revisit order on US-Canada border limits Media circles wagons for conspiracy theorist Neera Tanden Why the 'Never-Trumpers' flopped 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 JohnsonGraham: Trump will 'be helpful' to all Senate GOP incumbents Partisan headwinds threaten Capitol riot commission Cruz hires Trump campaign press aide as communications director MORE (R-Wis.), Rob PortmanRobert (Rob) Jones PortmanSunday shows preview: 2024 hopefuls gather at CPAC; House passes coronavirus relief; vaccine effort continues Grassley to vote against Tanden nomination Murkowski undecided on Tanden as nomination in limbo MORE (R-Ohio) and Richard BurrRichard Mauze BurrSenators given no timeline on removal of National Guard, Capitol fence Passage of the John Lewis Voting Rights Advancement Act is the first step to heal our democracy Overnight Health Care: COVID-19 vaccine makers pledge massive supply increase | Biden health nominee faces first Senate test | White House defends reopening of facility for migrant kids MORE (R-N.C.), all voted against the amendment.

Sen. Lisa MurkowskiLisa Ann MurkowskiGraham: Trump will 'be helpful' to all Senate GOP incumbents The Hill's Morning Report - Presented by The AIDS Institute - Senate ref axes minimum wage, House votes today on relief bill Republicans see Becerra as next target in confirmation wars 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.