Sen. Marco RubioMarco Antonio RubioHillicon Valley: Trump's Russia moves demoralize his team | Congress drops effort to block ZTE deal | Rosenstein warns of foreign influence threat | AT&T's latest 5G plans On The Money: Trump 'ready' for tariffs on all 0B in Chinese goods | Trump digs in on Fed criticism | Lawmakers drop plans to challenge Trump ZTE deal Overnight Defense: White House 'not considering' Ukraine referendum | Pompeo hopeful on plans for Putin visit | Measure to block ZTE deal dropped from defense bill MORE (R-Fla.) is endorsing Iowa Senate candidate Joni Ernst (R), giving her race a boost in the first-in-the-nation caucus state. 

The endorsement is the first time Rubio has stepped into a competitive GOP primary, and it could have repercussions not only for this race but for Rubio's presidential hopes should he decide to run in 2016. Ernst is facing big-spending businesssman Mark Jacobs (R), as well as former U.S. Attorney Matt Whitaker (R) and former conservative radio host Sam Clovis (R) in the primary.

"I'm proud to announce that Joni Ernst from Iowa is my Reclaim America PAC's newest endorsement of 2014," Rubio writes in an email to supporters, calling Ernst "the type of tough, no-nonsense conservative that we need in Washington."

Ernst has drawn endorsements from across the party. Former Massachusetts Gov. Mitt Romney (R) and former Alaska Gov. Sarah Palin (R) are both backing her, and she has quiet support from Iowa Gov. Terry Branstad (R) as well.

Rubio has been eyeing a presidential bid for some time, and if his candidate wins he could have an important ally in the early-voting state.