U.S. Senate Democrats made the opening move in their effort to portray Republicans as out of step with mainstream Americans on the question of whether humans cause climate change.

Democrats maneuvered a vote on an amendment offered by Hawaii Sen. Brian Schatz Wednesday that would designate climate change as a man-made event. Although the amendment was defeated, Republicans were forced to take a stand on the issue before the 2016 campaign begins.

The vote was intended to help determine “who the climate- change deniers in the U.S. Senate really are,” third-ranking Democrat Charles Schumer of New York said a day before the vote. “Do they deny that human activity has helped create climate change? Stay tuned — we’ll see.”

The Senate, by a 50-49 vote with 60 required, rejected the amendment to a Republican bill approving TransCanada Corp.’s Keystone XL oil pipeline. Republicans control the Senate 54-46.

The amendment would have deemed that “climate change is real” and that “human activity significantly contributes” to it.

South Dakota Senator John Thune, the chamber’s third- ranking Republican, said the proposal had political undertones.

“Obviously, it’s a very politically motivated vote,” he said. “They would love to get a bunch of Republicans voting against those amendments.”

Five Republicans voted in favor of Schatz’s amendment — Lamar Alexander of Tennessee, Kelly Ayotte of New Hampshire, Susan Collins of Maine, Lindsey Graham of South Carolina and Mark Kirk of Illinois. No Democrats voted against it.

Ten additional Republicans also voted for a separate proposal that said human activities contribute to climate change, though it didn’t call the contribution “significant.”

That amendment, designed to provide political cover to Republicans, was offered by Senator John Hoeven, a North Dakota Republican and the main sponsor of the Keystone pipeline bill. The amendment failed, 59-40.

Of the 15 Republicans, seven are up for re-election next year: Ayotte, Kirk, John McCain of Arizona, Lisa Murkowski of Alaska, Rand Paul of Kentucky, Rob Portman of Ohio and Pat Toomey of Pennsylvania.

Ayotte, Kirk, Portman and Toomey represent states that President Barack Obama won in 2008 and 2012.

“The American public understands that climate change is real and caused by humans,” Schatz said following the vote. “Disagreeing with the facts is going to be a disqualifier for most people in most states.”

Republicans next year must defend 24 Senate seats compared with 10 for Democrats, a reversal from the past two elections when significantly more Senate Democrats were on the ballot.

The climate-change votes probably will be packaged into Democratic fundraising pitches and election ads aimed at ousting Republicans who disagree that humans cause climate change.

Such an effort didn’t work last year when NextGen Climate Action Committee, billionaire environmentalist Tom Steyer’s super political action committee, funded an ad against Colorado Republican Corey Gardner.

The ad said Gardner “thinks he knows better than the scientists, NASA and the U.S. military on climate change.” Gardner defeated one-term Democratic Senator Mark Udall.

New York-based Kantar Media’s CMAG said 8,328 ads during the 2014 cycle had a “global warming” theme.

Senator Jim Inhofe of Oklahoma, the new chairman of the Environment and Public Works Committee, has written a book dismissing climate science as a hoax.

In a vote on a third amendment, Inhofe and 97 other senators agreed to assert that “climate change is real and not a hoax.” Senator Roger Wicker, a Mississippi Republican, was the only opponent of the amendment, offered by Senator Sheldon Whitehouse, a Rhode Island Democrat. The proposal didn’t address whether climate change is man-made, as did the other amendments.

Polls show that solid majorities of Americans think there’s evidence that global warming is occurring. A Yale University study showed that 56 percent of Republicans supported regulating carbon dioxide as a pollutant.

Sixty-four percent of Republicans backed tax rebates for people who buy energy-efficient vehicles or solar panels. The Yale study compiled data collected between March 2012 and October 2014.

Graham said Wednesday he accepts that global warming is happening and that it is largely man-made, but said the solutions pushed by Democrats would turn the economy “upside down.” He also said the Keystone XL pipeline has nothing to do with it, because the oil sands would be developed regardless whether the pipeline is built. Environmentalists, and their Democratic supporters, have cited global warming as a reason to oppose the $8 billion project. 

On Tuesday, Obama addressed the issue, saying in his State of the Union speech, “I’m not a scientist, either.”

However, he said, “The best scientists in the world are all telling us that our activities are changing the climate.”

Republicans will have at least one more opportunity to vote on climate science. Sen. Bernie Sanders, a Vermont independent, will offer a measure that says climate change is real and caused by human activities. But his measure adds that the U.S. needs to move away from fossil fuels as quickly as possible. 

Despite the outcomes Wednesday, Sen. Barbara Boxer of California called the votes a “breakthrough moment in the climate change debate.”

“We will go on the record, climate change is real and human activity contributes to climate change,” Boxer said before the vote. “What a breath of fresh air this amendment is.”

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The Associated Press contributed to this story.