The two Republicans have worked side by side since 2002 when both were elected to their first terms in office.
State Rep. Allen Icet of Wildwood, a St. Louis County community that falls in the 26th Senate District, announced his candidacy for state auditor in mid-June.
Icet was previously listed as a possible candidate for the Senate seat on the Missouri Ethics Commission Web site, which reports on candidates' campaign committees. Elected officials often continue to have a committee while in office to raise funds and be able to funnel funds left over from their own campaigns to others they support.
"The bottom line is I selected to stay right where am," Hillhouse said. "We have two credible candidates who have announced.
"I could either select to run for Senate and spend my energies there or I can spend my time doing what I was elected to do," he said, noting the county's road, building and infrastructure projects.
"More importantly I think we need to focus on economic development and try to attract jobs to the community," Hillhouse said.
Hillhouse's current term ends on Jan. 1, 2011.
"I could have tried to split my time but in fairness that's not what I should be doing. This is not the time," he said. "I'm not shying away from it.
"It's not from a lack of interest on the part of people out there," Hillhouse said.
"The decision to not run for U.S. Congress was easy," he said of his exploration into seeking the seat now held by U.S. Rep. Blaine Luetkemeyer.
"This decision I was torn between. I felt I would like to and was up to the challenge," Hillhouse said. "But I had to be pragmatic about the situation. I wouldn't feel comfortable running for a different office with so many issues unresolved at the county level."
Hillhouse said he was not closing the door on any future runs and announced he would seek re-election for presiding commissioner in 2010.
"I plan on staying right where I am," he said.
Hillhouse didn't say which of the two current candidates he would support or endorse.
Nieves previously told The Missourian he thought the race would come down to himself, Jackson and Hillhouse, but noted over a year stood between them and the primary.
The primary races are about 13 months away.
Sen. John Griesheimer, R-Washington, will be term-limited out of the Senate when his current term expires. The current senator and former county commissioner said he hopes to take a break from public office. He still has an active campaign committee according to the ethics commission Web site.
No Democrat has filed or announced they will run for Griesheimer's seat in the Republican-leaning district which consists of Franklin and Warren counties and part of St. Louis County.
The two Republican candidates are both facing term limits in the House. In Missouri officials can only serve eight years, or four terms, in the House and eight years, or two terms, in the Senate.
In all of Franklin County only two county-wide offices are held by Democrats. Second District Commissioner Ann Schroeder and Treasurer Alvin "Squeaky" Marquart are both members of the Democratic Party.
In the general election last year, 55 percent of county voters picked John McCain and Sarah Palin to fill the nation's highest offices. The same voters favored Democrat Jay Nixon for governor over Republican and former 9th U.S. Congressional District Rep. Kenny Hulshof, however.
