Cotton surprised his colleagues last week by exploiting a procedural loophole that allowed him to circumvent negotiations on Iran between McConnell (R-Ky.) and Senate Democratic Leader Harry Reid (Nev.).

In this case, Cotton was able to file an amendment to legislation granting Congress power to review a nuclear deal with Iran that would require Iran to recognize Israel. If the amendment is added to the bill, it would kill it, according to members in both parties.

The power play reflects Cotton’s emerging Senate role as a conservative check on McConnell, who many on the right suspect is too much of a dealmaker for his own good.

It also highlighted the tension between McConnell and his conservative critics.

McConnell doesn’t want Rubio’s Israel amendment to derail the Iran bill because he wants to rack up accomplishments to show the Senate GOP can govern. On Tuesday he listed trade legislation and the Iran measure as likely the two biggest accomplishments of the 114th Congress.