Democratic candidate Kyrsten Sinema speaks to supporters after officially winning the U.S. Senate race at the Omni Montelucia resort in Scottsdale, Arizona, U.S., November 12, 2018.

The Republican Party's difficulties in the traditionally red state of Arizona aren't likely to end with the election of Democrat Kyrsten Sinema to the U.S. Senate, according to a longtime political advisor to late Sen. John McCain.

Mark Salter, who was a confidant to and co-author for the Republican Arizona lawmaker and Washington icon for decades, responded to Sinema's victory, which became apparent nearly a week after Election Day, by breaking down what it meant for President Donald Trump and the GOP in the state.

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His prognosis isn't good for Republicans — particularly if they tie themselves tightly to Trump, as Sinema's opponent, Martha McSally, did during the campaign.

"Trump, who barely won AZ two years ago, is even more disliked there now," Salter wrote, suggesting it could get worse for the president when he is up for re-election in two years. Salter, like McCain, is known to be critical of Trump.

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The numbers back up Salter's argument about Trump's favorability and performance in the state. According to polling from Morning Consult, Trump had a net positive approval rating of 2 percentage points in Arizona in October, sharply down from 20 in January 2017, when he took office.

Trump won Arizona in 2016 by a margin of about 3.5 percentage points. It was a surprisingly small victory, given that Republicans have a strong presidential track record there. They have won the state in all but one election year dating to 1952. (Bill Clinton edged Bob Dole there in 1996.)