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 RubioRepublicans wary of US action on Iran California poll: Biden, Sanders lead Democratic field; Harris takes fifth The 13 Republicans needed to pass gun-control legislation MORE (R-Fla.), Ted CruzRafael (Ted) Edward CruzDemocrats seize Senate floor to protest gun inaction: 'Put up or shut up' Prospects for Trump gun deal grow dimmer Ted Cruz knocks New York Times for 'stunning' correction on Kavanaugh report MORE (R-Texas) and Rand PaulRandal (Rand) Howard PaulRepublicans wary of US action on Iran EXCLUSIVE: Swing-state voters oppose 'surprise' medical bill legislation, Trump pollster warns Rand Paul: Almost every mass shooter 'is sending off signals' 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 Kirk The 13 Republicans needed to pass gun-control legislation Advocates push for EpiPens on flights after college student's mid-flight allergic reaction Funding the fight against polio MORE (R-Ill.), Kelly AyotteKelly Ann AyotteGOP fears Trump backlash in suburbs Trump makes rare trip to Clinton state, hoping to win back New Hampshire Key endorsements: A who's who in early states MORE (R-N.H.), Lindsey GrahamLindsey Olin GrahamTrump fires back at Graham over Iran criticism Overnight Defense: GOP wary of action on Iran | Pence says US 'locked and loaded' to defend allies | Iran's leader rules out talks with US Republicans wary of US action on Iran MORE (R-S.C.), Lamar AlexanderAndrew (Lamar) Lamar AlexanderHere are the lawmakers who aren't seeking reelection in 2020 EXCLUSIVE: Swing-state voters oppose 'surprise' medical bill legislation, Trump pollster warns The 13 Republicans needed to pass gun-control legislation MORE (R-Tenn.) and Susan CollinsSusan Margaret CollinsThe Hill's Morning Report - Trump takes 2020 roadshow to New Mexico The 13 Republicans needed to pass gun-control legislation Congress passes bill to begin scenic byways renaissance 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 JohnsonRepublicans wary of US action on Iran Democratic senator warns O'Rourke AR-15 pledge could haunt party for years Conservatives offer stark warning to Trump, GOP on background checks MORE (R-Wis.), Rob PortmanRobert (Rob) Jones PortmanCost for last three government shutdowns estimated at billion The Hill's Morning Report - Trump takes 2020 roadshow to New Mexico The 13 Republicans needed to pass gun-control legislation MORE (R-Ohio) and Richard BurrRichard Mauze BurrLawmakers applaud Trump's ban on flavored e-cigarettes Trump to hold campaign rally in North Carolina day before special House election Hoekstra emerges as favorite for top intelligence post MORE (R-N.C.), all voted against the amendment.

Sen. Lisa MurkowskiLisa Ann MurkowskiKavanaugh impeachment push hits Capitol buzz saw McConnell lashes out at Democrats over 'unhinged' criticism of Kavanaugh The 13 Republicans needed to pass gun-control legislation 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.