"The fact is, everyone in Washington is just mad President [Donald] Trump didn't want to spend more of your money on foreign aid", Paul said via Twitter.
Paul said Trump usually makes decisions based on his instincts, which on foreign policy he described as "pretty good".
Trump is currently facing a pending impeachment trial in the US Senate after the House of Representatives voted last week to charge him with abuse of power and obstruction of justice for allegedly withholding security assistance to Ukraine.
US House Democrats launched in September an impeachment inquiry into the Trump-Zelensky phone call after a whistleblower submitted a complaint alleging the US president abused the power of the office. The complaint claimed Trump pressured Zelensky to investigate his political rival, Joe Biden, and his son, Hunter, for corruption and used congressionally approved military aid for Ukraine as leverage.
Trump has denied any wrongdoing, repeatedly dismissing the impeachment inquiry as a witch hunt aimed at reversing the outcome of the 2016 presidential election.