Sen. Marco Rubio (R-FL) attended a fundraiser for Sen. Jeff Flake (R-AZ) on Monday in a bid to support his 2018 Senate reelection campaign against challenger Kelli Ward.

Rubio was the headline speaker at the exclusive event in Scottsdale, Arizona, for which lunch tickets cost $100 and VIP tickets cost $500.

Both Rubio and Flake were part of the “Gang of Eight” that proposed the 2013 immigration bill that sought to provide amnesty to millions of illegal immigrants. Flake subsequently endorsed Rubio’s failed 2016 presidential campaign.

When asked by Breitbart News why he endorsed Flake, Rubio’s office did not return a request for comment.

Flake has been a prominent critic of President Donald Trump from the outset, having refused to back his presidential campaign and regularly attacking the policies of his administration, particularly those regarding immigration. Flake proudly describes himself as a “globalist.”

Meanwhile, Trump has already signaled his support for Flake’s primary challenger Kelli Ward, describing Flake as “weak on borders” and “toxic.”

Great to see that Dr. Kelli Ward is running against Flake Jeff Flake, who is WEAK on borders, crime and a non-factor in Senate. He's toxic! — Donald J. Trump (@realDonaldTrump) August 17, 2017

A poll conducted by JMC Analytics in August found that Ward already has a 25-point lead against Flake more than a year before the election, currently polling at 47 percent support compared to Flake’s 21 percent.

Follow Ben Kew on Facebook, Twitter at @ben_kew, or email him at bkew@breitbart.com.