Eight days before Election Day, the "momentum is on the Republicans' side" in key Senate races, Fox News contributor Karl Rove said.

Appearing on Fox News' "America's Newsroom" on Monday, the former George W. Bush adviser predicted Republican victories in Arkansas and Kansas and "very close" races in North Carolina and Colorado.

In Arkansas, Republican Tom Cotton leads incumbent Democrat Mark Pryor in a race that has "every sense of firming up," according to Rove. Cotton has the edge by 2 points in a NBC News/Marist Poll and 5 points in a Real Clear Politics average.

In Kansas, Republican incumbent Sen. Pat Roberts narrowly trails independent challenger Greg Orman in a race that Rove says shows "clear momentum" for Republicans.

"[Roberts] was down 10 in early October. It's down one now," he said.

In North Carolina, Republican Thom Tillis trails Democratic incumbent Kay Hagan by less than 2 points in the latest Real Clear Politics average.

"The movement has been on [Tillis'] side," Rove said. "The question is going to be, is it going to be fast enough and sustained enough?"

In Colorado, early mail-in ballot returns are heavily favoring Republicans compared with 2010 as challenger Cory Gardner looks to unseat Democrat Mark Udall.

"It's been a seven-point swing towards Gardner," Rove said of two recent NBC polls. "This is a state where there's been a durable, consistent pattern of Gardner having the lead."

Rove sees Republicans gaining six to eight seats total, but said the ground game and voter turnout will play a large role on Nov. 4.

"Who actually turns out on Election Day matters a lot," he said. "African-American turnout in Georgia, for example. Hispanic turnout in Colorado. These are critical for Democrats."