Donald Trump is hemorrhaging support in Wisconsin by refusing to endorse House Speaker Paul Ryan, a Republican lawmaker from the battleground state said Thursday.

"Paul Ryan is loved in our state because he's a conservative who has advocated for conservative policies, and Donald Trump coming out saying favorable things about Mr. Ryan's opponent doesn't add to the number of voters in Wisconsin that will vote Donald Trump," Wisconsin Rep. Sean Duffy told CNN's "New Day."

Duffy noted that Trump behaved similarly in the GOP primary by attacking Wisconsin Gov. Scott Walker days before voters in the Badger State cast their ballots. Trump ultimately lost Wisconsin to Texas Sen. Ted Cruz.

"You go after a Scott Walker and a Paul Ryan, you don't add to the number of voters that you need to win Wisconsin — you detract from them. Big mistake," Duffy said. "Paul Ryan will win and we'll get beyond this after next Tuesday, but I don't think it's very smart on [behalf] of Trump."

He continued, "We need to add people to the Republican Party, those independents who can swing either way, and this is only detracting."

Duffy, who praised Trump last month in a speech at the GOP convention, suggested that perhaps the Republican presidential nominee lacks an "understanding of what we are in Wisconsin."

"I can't get in the mind of Donald Trump and tell you why he's doing it. Paul Ryan is the gold standard for conservatism. I don't know what Donald Trump is doing. ... It makes no sense," he said.

Trump is slated to hold a rally in Green Bay on Friday. Walker, Ryan and Wisconsin Sen. Ron Johnson have all declined to attend.