Rubio’s complex history on immigration will be back in the spotlight when the GOP-led Senate takes up the sanctuary cities measure, which was spearheaded by Sen. David Vitter (R-La.), one of the most hard-line opponents of looser immigration policies. Sanctuary cities are localities that decline to cooperate with federal immigration authorities, believing their policies can invite racial profiling and harm local policing strategies. When Vitter rolled out the bill earlier this month, Rubio was one of more than a half-dozen Republican co-sponsors.

The measure on the Senate floor Tuesday would impose a five-year mandatory minimum prison sentence for some immigrants who repeatedly try to enter the United States illegally. Vitter and other conservative Senate Republicans — such as Texas Sen. Ted Cruz — have been on the attack against sanctuary cities since July, when authorities say a Mexican immigrant in the country illegally shot and killed a young woman in San Francisco. Most Senate Democrats will vote to block it, aides said Monday.

Understanding Rubio’s immigration views requires grasping his positions on both policy and procedure.

Rubio has not disavowed his stances on immigration during his presidential campaign.