Sen. Marco RubioMarco Antonio RubioRubio: McCabe 'should've been allowed to finish through the weekend' For Tillerson, bucking Trump became a job-killer At least six dead after pedestrian bridge collapses on cars in Florida MORE (R-Fla.) on Tuesday backed freshman Rep. Tom CottonThomas (Tom) Bryant CottonRussian spy poisoning brings world powers closer to day of reckoning GOP senators see Tillerson ouster as the new normal Cotton: Russia will 'lie and deny' about British spy poisoning MORE (R-Ark.) for the Senate in his first endorsement of the 2014 cycle.

The endorsement comes with a twist, as Cotton is a strident opponent of the bipartisan immigration bill that Rubio helped pass through the Senate.

Rubio said Cotton would stick to his guns if elected to the Senate from Arkansas.

"Once elected, we can be confident that Tom will be another conservative friend in Washington who will ensure that our children and grandchildren inherit an exceptional America from this generation," Rubio said in an email from his leadership PAC.

"He won't stand down when it comes to repealing ObamaCare, fighting to lower our runaway debt, and turning back the tide of big government that has taken over Washington in recent years."

Rubio and Cotton have similar political profiles.

Both are staunch fiscal conservatives bouyed by the Tea Party movement, and have been embraced by Washington's GOP establishment.

The two men also have hawkish views on foreign policy that are at odds with the libertarian streak that has taken hold in the GOP.

Cotton is running against Sen. Mark PryorMark Lunsford PryorMedicaid rollback looms for GOP senators in 2020 Cotton pitches anti-Democrat message to SC delegation Ex-Sen. Kay Hagan joins lobby firm MORE (D-Ark.) in a must-win race for Senate Republicans if they hope to regain control of the upper chamber.