With most predicting he will go down in defeat, Democratic Sen. Russ Feingold earned compliments from his former GOP ally, Sen. John McCainJohn Sidney McCainSenate panel advances 6B defense policy bill McCain: Trump pardoning Jack Johnson 'closes a shameful chapter in our nation’s history' Trump pardons late boxing champion Jack Johnson MORE, on Monday.

The Arizona senator and 2008 GOP presidential nominee has campaigned hard for Republican Senate candidates this cycle while sharply criticizing their Democratic rivals, such as Sens. Barbara BoxerBarbara Levy BoxerThe ‘bang for the buck’ theory fueling Trump’s infrastructure plan Kamala Harris endorses Gavin Newsom for California governor Dems face hard choice for State of the Union response MORE (Calif.) and Patty MurrayPatricia (Patty) Lynn MurrayOvernight Health Care — Sponsored by PCMA — Trump official won't OK lifetime limits on Medicaid Dems warn against changes to federal family planning program Overnight Health Care: Drug company under scrutiny for Michael Cohen payments | New Ebola outbreak | FDA addresses EpiPen shortage MORE (Wash.) But McCain saved kind words for Feingold (Wis.), with whom he worked on landmark campaign finance reform legislation eight years ago.

Asked on Fox News if he feels bad when his Senate colleagues lose, McCain said, "Yes, and if I may say so, I've grown to have the greatest respect and affection for my friend, Russ Feingold. He's an honest man, a man of great integrity, and I've grown to appreciate him more than ever. And it looks like he might be a casualty tomorrow."

McCain has taken his lumps from critics ever since his presidential campaign for moving to the right instead of championing his bipartisan work in the upper chamber with members like Feingold. 

Now the three-term Wisconsin senator is poised to lose to upstart GOP candidate Ron JohnsonRonald (Ron) Harold JohnsonHouse GOP sets three FBI interviews in Clinton probe Overnight Health Care — Sponsored by PCMA — House passes 'right to try' drug bill | Trump moves to restrict abortion referrals House approves 'right to try,' sends bill to Trump's desk MORE, who leads by high single digits in most polls.

Johnson did not receive an endorsement from McCain, nor did the Arizona senator donate to his campaign.

McCain in January decried the Citizens United Supreme Court decision for knocking down much of the law he passed with Feingold. But so far he has not signed on to legislation requiring greater disclosure for corporate and union political spending.