With most predicting he will go down in defeat, Democratic Sen. Russ Feingold earned compliments from his former GOP ally, Sen. John McCainJohn Sidney McCainLawmakers worry about rise of fake video technology Democrats put Dreamers and their party in danger by playing hardball Trump set a good defense budget, but here is how to make it better 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 BoxerKamala Harris endorses Gavin Newsom for California governor Dems face hard choice for State of the Union response Billionaire Steyer to push for Dem House push MORE (Calif.) and Patty MurrayPatricia (Patty) Lynn Murray30 million people will experience eating disorders — the CDC needs to help Mulvaney remarks on Trump budget plan spark confusion Overnight Finance: Mulvaney sparks confusion with budget remarks | Trump spars with lawmakers on tariffs | Treasury looks to kill 300 tax regs | Intel chief's warning on debt 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 JohnsonTrump spars with GOP lawmakers on steel tariffs Overnight Regulation: Trump unveils budget | Sharp cuts proposed for EPA, HHS | Trump aims to speed environmental reviews | Officials propose repealing most of methane leak rule Trump budget seeks savings through ObamaCare repeal 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.