Four people were arrested at Sen. Tom CarperThomas (Tom) Richard CarperHillicon Valley: Facebook, Google struggle to block terrorist content | Cambridge Analytica declares bankruptcy in US | Company exposed phone location data | Apple starts paying back taxes to Ireland Overnight Energy: Pruitt taps man behind 'lock her up' chant for EPA office | Watchdog to review EPA email policies | Three Republicans join climate caucus Watchdog to probe EPA email preservation MORE’s (D-Del.) Washington, D.C. office on Tuesday during a protest over the Keystone pipeline.

A Carper aide confirmed that the individuals were protesting his planned vote to approve the controversial oil pipeline.

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Capitol Police spokesman Shennell Antrobus said officers were called earlier in the afternoon to Carper’s office in the Hart Senate Office Building.

Four individuals were arrested and charged with “crowding, obstructing or incommoding.” Antrobus did not identify the individuals arrested.

Carper has said he will vote to approve the oil pipeline when a measure comes up for a vote Tuesday evening. The senator is a supporter of environmental policies and his position came as a surprise to some green groups.

Carper said he believes Congress cannot address broader questions about climate change or pollution until they first deal with Keystone.

“We need to address this issue and we need to move on,” he told the Wilmington News Journal.

The Keystone issue has taken on added urgency because of the close Senate race in Louisiana. Sen. Mary LandrieuMary Loretta LandrieuLandrieu dynasty faces a pause in Louisiana Senate GOP rejects Trump’s call to go big on gun legislation Project Veritas at risk of losing fundraising license in New York, AG warns MORE (D-La.) is sponsoring the Senate bill, and her runoff challenger, Rep. Bill Cassidy (R-La.), sponsored a similar measure that passed the House last week. Both are touting their energy credentials in their race.

The vote on the Senate bill is expected to be close, with Landrieu just short of the 60 votes needed for approval.

The White House has suggested that President Obama is considering a veto if the bill reaches his desk.

"I’m not in a position to issue veto threats from here, but … there are similar pieces of legislation that have been introduced in this Congress where the president’s senior advisers have recommended a veto,” White House press secretary Josh Earnest said on Tuesday.