The early and unofficial returns show the hottest state Senate race is so far favoring the Republican candidate.

The race to watch on Tuesday night is Senate District 5, which is based in the central portion of the Western Slope and where Republican Rep. Marc Catlin of Montrose faces Glenwood Springs Democrat Cole Buerger. The contest saw the most spending by outside groups on any Senate race, with more than $3 million spent in October alone.

As of 9 p.m., Catlin held a lead over Buerger by nearly 2,100 votes or about 3 percentage points. Buerger was doing well in Garfield, Eagle, Pitkin and Gunnison counties; Catlin was performing well in Delta and Montrose and Hinsdale counties. 

The 35-member Senate has 18 seats up for election Tuesday night. Nine were open seats, meaning there were no incumbents running for them.

Seven seats turned over because of term-limits, including that of Senate President Steve Fenberg, D-Boulder, as well as the following senators: Rhonda Fields, D-Aurora; Bob Gardner, R-Colorado Springs; Joann Ginal, D-Fort Collins; Kevin Priola, D-Henderson; Jim Smallwood, R-Parker; and, Rachel Zenzinger, D-Arvada.

Sen. Perry Will, R-New Castle, declined to run for his Senate District 5 seat, instead choosing to seek a position on the Garfield County Board of County Commissioners. Sen. Kevin Van Winkle, D-Douglas County, also chose not to run for his Senate District 30 seat, instead vying for a post on the Douglas County Board of County Commissioners.

All of those positions were in safe districts for their parties, except for Gardner, whose seat was one of the most competitive in 2024.

Outside expenditure groups also waged a pitched battle in several districts. Through Oct. 28, the Senate Majority Fund, which favors Republican candidates, held a lead of more than $1 million.

Meanwhile, Gardner’s Senate District 12 seat saw a contest between Democratic Rep. Marc Snyder of Manitou Springs and Republican Stan VanderWerf of Colorado Springs. Snyder was expected to win the central Colorado Springs district, which the redistricting commission showed a 2.4% Republican lean in 2021.

As of 9 p.m., Snyder held a lead of nearly 3 percentage points, or just under 1,400 votes, over VanderWerf.

Sen. Cleave Simpson, R-Alamosa, faced Democratic candidate Vivian Smotherman of Durango in Senate District 6, which represents the southern portion of the Western Slope, east to Alamosa. 

As of 9 p.m., Simpson held a commanding lead of 9 percentage points or more than 8,000 votes. He led in the San Luis Valley and in nine of the district’s 14 counties. Smotherman was leading in four counties, including her home county of La Plata.

In Senate District 16, Sen. Chris Kolker, D-Centennial, faced off against Republican Robyn Carnes.

The early, unofficial returns at 9 p.m. showed Kolker leading Carnes by a 5 percentage points or more than 4,000 votes.

Kolker won his first term in 2020 by more than 10 percentage points over former Assistant Secretary of State and Republican Suzanne Staiert. In 2020, when Kolker won his first election in then-Senate District 27, the district was exclusively located in Arapahoe County. Redistricting moved part of southeastern Jefferson County into the political boundary, which is rated as having a 4.7% Democratic lean, according to the redistricting commission’s analysis.

One seat viewed as most likely to flip is Senate District 13, which is located in Weld and a small portion of Adams County. Term-limited Priola had switched parties in 2022 but the seat was expected to go back to Republicans this year, with Republican Scott Bright facing Democratic candidate Matt Johnson.

Not surprisingly, Bright held a substantial lead of more than 12 percentage points over Johnson, based on the unofficial returns as of 9 p.m. from both Adams and Weld counties.