DENVER (AP) – Republican plans to take control of one or both chambers of Congress go straight through Colorado this Election Day, where the GOP is fighting to pick off a senator and three House Democrats.

Colorado voters are choosing whether to retain appointed Democratic Sen. Michael Bennet and seven members of the House. Three of those House incumbents are fighting off well-funded Republican challengers.

Bennet and Republican Senate candidate Ken Buck both spent Monday concentrating on the Front Range, where most Colorado voters live. Bennet stopped in Colorado Springs, Denver, Boulder and Fort Collins, while Buck took a similar path in the opposite direction, from Fort Collins south to the Denver suburbs.

Out-of-state political advocacy groups have spent more on Colorado’s Senate race than any other Senate race this year, more than $32 million. Combined with what the candidates have spent themselves, the race tops $40 million. Polls indicate it could go either way.

If Bennet wins, Democrats will keep control of the state’s Senate delegation for the first time since the 1970s. If Buck wins, Republicans will overcome a candidate who has White House backing and a dramatic fundraising advantage.

Three Democratic House members are fighting for their jobs: Rep. Betsy Markey in northern Colorado; Rep. John Salazar in western Colorado; and Rep. Ed Perlmutter in the Denver suburbs.

Markey, long considered the most vulnerable incumbent in Colorado, is seeking a second term in a district that includes more Republicans than Democrats.

Markey spent her final days campaigning in the most populous parts of her district, Larimer and Weld counties. She hopes to run up big numbers in those areas because she’s likely to lose badly in the heavily Republican eastern parts of the district to Republican Cory Gardner.

Salazar spent his final campaign days concentrating on where he’s strongest: Southern Colorado. Salazar campaigned with his younger brother, Interior Secretary Ken Salazar, and New Mexico Gov. Bill Richardson. John Salazar’s challenger, Republican state Rep. Scott Tipton, spent his final days in mountain towns and planned to spend Election Night in the Republican stronghold of Grand Junction.

Perlmutter campaigned with Democratic Sen. Mark Udall and Democrat Andrew Romanoff, who unsuccessfully challenged Bennet in a Democratic primary last summer. His opponent, Ryan Frazier, has attracted big spending from outside interests. Pro-Republican groups including the American Action Network have spent more in this district than any other in Colorado.

Four members of Colorado’s congressional delegation – Democrats Diana DeGette and Jared Polis, and Republicans Mike Coffman and Doug Lamborn – faced only nominal opposition Tuesday.