Sen. Cory Gardner (R-Colorado) said he opposed President Donald Trump's emergency declaration until it was announced. Thursday, he voted for it.

KUSA – Sen. Cory Gardner (R-Colorado) was not one of the 12 Republicans who voted with Democrats to reject President Donald Trump’s declaration of a national emergency at the Mexico border.

This rebuke of the president’s proclamation passed the Senate 59-41 Thursday morning, and could lead to a showdown between Congress and Trump.

Gardner’s vote comes two weeks after his spokesperson told 9NEWS the senator was unsure if he would vote against the national emergency declaration, which was supposed to involve spending more than $3.6 billion more for border barriers.

Trump first declared the emergency in February to provide funding for a border wall with Mexico – something he promised during his 2016 campaign.

Gardner’s vote for Trump’s national emergency declaration came contrary to previous statements in which he said he said he personally told the president he opposes it.

Trump’s response to the Senate vote came in the form of a tweet that simply said “VETO!”

"I'll do a veto. It's not going to be overturned," Trump told reporters in the Oval Office earlier in the day. "It's a border security vote."

Gardner is up for reelection in 2020 and is in some circles considered one of the most vulnerable Republican senators. His Senate counterpart in Colorado, Democrat Michael Bennet, voted along party lines to reject the president’s emergency declaration.

In a measure of how remarkable the confrontation was, Thursday was the first time Congress has voted to block a presidential emergency since the National Emergency Act became law in 1976.

The Associated Press contributed to this report.