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 RubioDemocrats cool on Crist's latest bid for Florida governor Tim Scott sparks buzz in crowded field of White House hopefuls The unflappable Liz Cheney: Why Trump Republicans have struggled to crush her  MORE (R-Fla.), Ted CruzRafael (Ted) Edward CruzFormer CEO Glenn Youngkin wins Virginia GOP gubernatorial convention The Memo: Outrage rises among liberals over Israel Cheney drama exposes GOP's Trump rifts MORE (R-Texas) and Rand PaulRandal (Rand) Howard PaulTim Scott sparks buzz in crowded field of White House hopefuls Sherrod Brown calls Rand Paul 'kind of a lunatic' for not wearing mask Overnight Health Care: WHO-backed Covax gets a boost from Moderna 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 KirkDuckworth announces reelection bid Brave new world: Why we need a Senate Human Rights Commission  Senate majority battle snags Biden Cabinet hopefuls MORE (R-Ill.), Kelly AyotteKelly Ann AyotteSununu seen as top recruit in GOP bid to reclaim Senate Lobbying world Overnight Defense: NATO expanding troops in Iraq MORE (R-N.H.), Lindsey GrahamLindsey Olin GrahamHouse conservatives take aim at Schumer-led bipartisan China bill There will be no new immigration law under Biden, unless he changes course McConnell safe in power, despite Trump's wrath MORE (R-S.C.), Lamar AlexanderLamar AlexanderThe Republicans' deep dive into nativism Senate GOP faces retirement brain drain The Hill's Morning Report - Presented by the National Shooting Sports Foundation - CDC news on gatherings a step toward normality MORE (R-Tenn.) and Susan CollinsSusan Margaret CollinsTop female GOP senator compares Cheney ousting to 'cancel culture' Utah county GOP censures Romney over Trump impeachment vote House conservatives take aim at Schumer-led bipartisan China bill 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 JohnsonSunday shows preview: Coronavirus dominates as White House continues to push vaccination effort Overnight Health Care: WHO-backed Covax gets a boost from Moderna Vaccine hesitancy among lawmakers slows return to normalcy on Capitol Hill MORE (R-Wis.), Rob PortmanRobert (Rob) Jones PortmanHouse conservatives take aim at Schumer-led bipartisan China bill Strengthen CBP regulations to reduce opioid deaths House panel advances bipartisan retirement savings bill MORE (R-Ohio) and Richard BurrRichard Mauze BurrUtah county GOP censures Romney over Trump impeachment vote Battle lines drawn over Biden's support for vaccine waivers FDA unveils plan to ban menthol cigarettes, flavored cigars MORE (R-N.C.), all voted against the amendment.

Sen. Lisa MurkowskiLisa Ann MurkowskiUtah county GOP censures Romney over Trump impeachment vote Bottom line Pollster Frank Luntz: 'I would bet on' Trump being 2024 GOP nominee 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.