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 RubioSenate confirms Trump’s pick to lead NASA The Hill's 12:30 Report Steps Congress can take to defend America against foreign influence operations MORE (R-Fla.), Ted CruzRafael (Ted) Edward Cruz32 male senators back Senate women's calls to change harassment rules Senate confirms Trump’s pick to lead NASA DOJ denies reports judicial nominee once called illegal immigrants 'maggots' MORE (R-Texas) and Rand PaulRandal (Rand) Howard PaulHeitkamp becomes first Dem to back Pompeo for secretary of State Senate committee sets Monday vote even as Pompeo appears to lack support Trump checkmates Democrats in sending Pompeo to North Korea 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 KirkHigh stakes as Trump heads to Hill Five things to watch for at Trump-Senate GOP meeting Giffords, Scalise highlight party differences on guns MORE (R-Ill.), Kelly AyotteKelly Ann AyotteAudit finds US Defense Department wasted hundreds of millions of taxpayer dollars US sends A-10 squadron to Afghanistan for first time in three years No, the US did not spend million on a gas station in Afghanistan MORE (R-N.H.), Lindsey GrahamLindsey Olin GrahamDems walk tightrope on Pompeo nomination Senate panel moving ahead with Mueller bill despite McConnell opposition Overnight Defense: Lawmakers worry over Syria strategy | Trump's base critical of strikes | Flake undecided on Pompeo | Coast Guard plans to keep allowing transgender members | GOP chair wants to cut B from Pentagon agencies MORE (R-S.C.), Lamar AlexanderAndrew (Lamar) Lamar AlexanderMaternal deaths keep rising in US, raising scrutiny Supreme Court weighs future of online sales taxes Senators press administration on mental health parity MORE (R-Tenn.) and Susan CollinsSusan Margaret CollinsOvernight Energy: Trump NASA pick advances after drama | White House office to investigate Pruitt's soundproof booth | 170 lawmakers call for Pruitt to resign Trump's NASA nominee advances after floor drama Family, friends mourn death of Barbara Bush 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 JohnsonGOP senator: Congress needs ‘to move on’ from Russia probe GOP senator: ‘Way too early’ to talk about supporting Trump in 2020 IG report faults fired FBI official McCabe for leak to media MORE (R-Wis.), Rob PortmanRobert (Rob) Jones PortmanTax rules will be subject to more OMB review under new memo Ending sex trafficking tomorrow requires preventing child abuse today Doctors bristle at push for opioid prescription limits MORE (R-Ohio) and Richard BurrRichard Mauze BurrSenators express concerns over Haspel's 'destruction of evidence' Overnight Cybersecurity: US, UK blame Russia for global cyberattacks | Top cyber official leaving White House | Zuckerberg to meet EU digital chief Senators, state officials to meet on election cybersecurity bill MORE (R-N.C.), all voted against the amendment.

Sen. Lisa MurkowskiLisa Ann MurkowskiSenators press administration on mental health parity Overnight Energy: Watchdogs unveil findings on EPA, Interior controversies | GAO says EPA violated law with soundproof booth | IG says Zinke could have avoided charter flight | GOP chair probes Pruitt's four email addresses GOP fractures over push to protect Russia probe 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.