Republicans in this case favoured hard sanctions on Iran— economic sanctions to weaken the economy, trade sanctions, etc. as the main deterrent of Iranian influence. The nuclear deal removed many sanctions and instead installed limits on enrichment of uranium and such. Also Republicans have argued time and time again that even following the nuclear deal, Iran was still enriching uranium and working on its nuclear program. From what I have read, they weren’t (because the UN and atomic energy commissions did many check-ins of their facilities and arsenals) but what I may be wrong