Marco Rubio started off the year as one of the most popular Republicans among the GOP, and according to the latest poll from Rasmussen, he’s still more popular than not. After several months of working on immigration reform, though, Rubio’s numbers have slid significantly within his party — by fifteen points (via Byron York):

Rubio, the most visible advocate of the Senate’s comprehensive immigration reform plan, is now viewed favorably by 58% of Republican voters nationwide. That down 10 points since May and 15 points since February. A new Rasmussen Reports national telephone survey finds that 16% of GOP voters have an unfavorable opinion of him, while 25% are undecided. (To see survey question wording, click here.) The level of enthusiasm for Rubio has declined significantly. Today, just 21% of GOP voters have a Very Favorable opinion of the Florida senator. That’s down sharply from 44% in February and 31% in May. Among all voters, 37% view Rubio favorably, down five points from February, while 35% have an unfavorable opinion of the first-term senator. Twenty-seven percent (27%) are undecided.

The reaction from overall voters hasn’t actually changed much since May, when Rubio was at 39/32. That change is within the margin of error. The reaction among GOP voters has been a little more significant over the last six weeks. In May, Rubio got a 68/10 rating, but now it’s 58/16.

However, a 58/16 is still not exactly a damning favorability rating. There are a lot of 2016 wannabes that wouldn’t mind trading Rubio for those numbers. Six weeks ago, Ted Cruz got a 42/12, and now Rand Paul has a 63/19, but Paul has a 37/43 with overall voters. Among independents, Rubio gets a 41/35 to Paul’s 37/37.

The trend line has to be worrisome (assuming that Rubio’s interested in running for anything more than Senator in 2016), but at a certain point the immigration issue will become fixed in those ratings. So far, it looks as though Rubio has a long way to fall, and not much more downside to risk.