Sen. Rand Paul is lending Democrats a hand in their effort to block President Donald Trump’s national emergency declaration. Over the weekend the Kentucky lawmaker became the fourth Republican senator to say he would vote in favor of a resolution that would block Trump’s measure to build a border wall through an emergency declaration. “I can’t vote to give the president the power to spend money that hasn’t been appropriated by Congress,” Paul said at an event in Kentucky, according to the Bowling Green Daily News. “We may want more money for border security, but Congress didn’t authorize it. If we take away those checks and balances, it’s a dangerous thing.”

Republican Sens. Susan Collins (Maine), Lisa Murkowski (Alaska) and Thom Tillis (N.C.) had previously said they would vote in favor of the resolution to block Trump’s national emergency declaration. With Paul’s vote the resolution would pass when it is set to reach the Senate later this month. The House passed the resolution on Tuesday in a 245-182 vote with the support of 13 Republicans. Trump has vowed to veto any resolution that would block his emergency declaration. It would be the first veto of his presidency but would almost certainly assure he would get his way since lawmakers wouldn’t have the votes to override the veto.

Around a dozen Republican senators are still undecided and more could speak up now that Paul made the announcement. “Paul’s decision to become the deciding vote could free other senators to voice their opposition,” notes Politico. “A significant bloc of Republicans dislike the national emergency, though some senators question whether that will translate to ‘yes’ votes on the disapproval resolution.” Some senators have been wary of breaking with the president on border security issues even if they disagree with the national emergency declaration. That is why many of those who are supporting the resolution are framing the opposition in terms of separation of powers rather than characterizing it as a vote either for or against Trump.

It’s unclear when the Senate will vote on the resolution. Officially, the Senate has 18 days from the date the House approved the resolution to carry out its own vote.