Koster was first elected to the Missouri Senate in 2004 as a Republican. He represented Missouri's 31st Senatorial District, which consists of Cass, Johnson, Bates and Vernon counties. During his time in the Missouri General Assembly, Koster played key roles in the debates over stem cell research, tort reform, and the elimination of Medicaid fraud. In 2006, he carried legislation in the Senate that overhauled Missouri’s eminent domain laws. He served on these Senate committees:
Economic Development, Tourism, and Local Government
Agriculture, Conservation, Parks and Natural Resources
Koster championed legislation for Truth in the Missouri Court System, and proposed a bill that aimed to eliminate paternity fraud. On August 1, 2007, Koster made Missouri political history when he announced that he was leaving the Missouri Republican Party to become a Democrat. Citing his longstanding differences with the Republican Party on issues like stem cell research, workers' rights, and the non-partisan court plan, Koster said that the Missouri Republican Party had become too beholden to the extreme right-wing to lead the state of Missouri. He said, "Today, Republican moderates are all but extinct." Before his change of parties, Koster was chairman of the Republican Caucus, the majority party's fourth-ranking position in the Missouri State Senate.
Attorney General
On August 5, 2008, Koster narrowly defeated State Representative Margaret Donnelly in the Democratic primary for the nomination for Missouri Attorney General. Koster won despite accusations that his campaign violated state law in raising money from multiple committees. He also survived the disclosure that he played a supporting role in a plagiarism episode that damaged Attorney General William L. Webster’s campaign for governor in 1992. Fresh out of law school, Koster worked for Webster, a Republican, as an assistant state attorney general. His campaign was not easily won because he had to overcome the label of "opportunist" as a result of switching parties during the '08 election. He then went on to defeat Republican State Senator Mike Gibbons in the general election, 52.83% to 47.17%. He was sworn in as attorney general on January 12, 2009, succeeding Jay Nixon. Koster is an advocate of the death penalty, and as of July 2013, there were 21 inmates on death row in Missouri whose executions he was pressing the Supreme Court of Missouri to expedite. After the 21 inmates filed suit before the state Supreme Court against the Missouri Department of Corrections over the use of the drug propofol used in lethal injections, concerning cruel and unusual punishment, the state Supreme Court temporarily halted the further use of the death penalty until the case had been ultimately decided. With Missouri law allowing for the use of other forms of the death penalty in cases where lethal injection is not available, Koster has advocated the use of gas chambers to execute Missouri prisoners. In 2012, after Koster's staff of 56 moved to a portion of the Broadway Building in Jefferson City, a $3.2 million request was made for "repairs, replacements and improvements" on two floors, "to include interior demolition, construction of new interior finishes, upgrades to the HVAC and electrical systems, new furniture, fixtures and equipment." Koster's own office is in the Supreme Court Building. "Funding the proposed renovation would not come from general revenue, but rather about $400,000 in administrative allowances that accompany federal grants, and about $2.8 million from the Merchandising Practices Revolving Fund. Money recovered by the state in consumer fraud cases goes into the fund, which is available to pay expenses of the attorney general's office." Koster supports same-sex marriage, but defended his state's former constitutional ban on it because voters approved it. Koster's office defended U.S. District Judge Catherine D. Perry after she denied a motion for temporary restraining orders on six police officers enforcing a "5-second rule" that required demonstrators to move every five seconds or face arrest in Ferguson, Missouri, citing the need for law enforcement's protection of property and the availability of a "free-speech zone". However, at the time of this ruling, the free speech zone was off-limits to the public. This "5-second rule" was later determined to be unconstitutional by a different federal judge. In October 2014, a California judge dismissed a lawsuit Koster filed, rejecting the arguments of six states that challenged California's prohibition on the sale of eggs laid by caged hens kept in conditions more restrictive than those approved by California voters in a 2008 ballot initiative, Proposition 2. Judge Kimberly Mueller ruled that the states lacked legal standing to sue on behalf of their residents and that Koster and other plaintiffs were representing only the interests of egg farmers, rather than "a substantial statement of their populations". According to the Kansas City Star. Koster's office spent more than $83,000 on the failed lawsuit.
2016 gubernatorial election
As his second term as attorney general was coming to an end, Koster was running for governor. He easily won the August 2 Democratic primary and faced Republican Eric Greitens in the November 8 general election. He lost the general election with 45.4% of the vote to Greitens' 51.3%.
Controversies
Missourians for Honest Leadership: In 2008, Koster's ex-wife Rebecca Bowman Nassikas donated $200,000 to a dormant political committee, Missourians for Honest Leadership, which paid $187,500 to purchase air time and produce television ads in an effort to oppose Koster's candidacy for Missouri Attorney General.
Contingency Contracts: A 2012 audit by state Auditor Tom Schweich criticized Koster for his practice of awarding contingency fee contracts to law firms that had donated hundreds of thousands of dollars to his campaign.
Pay to Play allegations: In October 2014, House Speaker Tim Jones announced plans to investigate charges that Koster took actions in office that were designed to benefit campaign contributors. News coverage revealed that The Simmons Firm had donated two $50,000 checks to Koster's campaign in 2013 only months after Koster filed a lawsuit against Republic Services over its underground fire at the Bridgeton Landfill.; that Koster had ended an inquiry focusing on the company 5-hour Energy after conversations with a lobbyist for the company who was also a Koster contributor; and that Koster had negotiated an agreement with Pfizer, another campaign contributor, to pay Missouri $750,000 in connection with a multistate investigation of illegal marketing practices, about $350,000 less than what the state would have collected had it participated in a joint negotiation with other states, and attended a Pfizer convention as a speaker and guest during the settlement negotiation.
Koster ultimately said that he would no longer accept gifts from lobbyists. In Missouri, it is legal for elected officials to accept unlimited campaign contributions and gifts from lobbyists, and despite his position on lobbying reform, Koster rejects the idea of placing limits on the amount of money a corporation or a wealthy individual could contribute to a campaign. He instead suggests adding more transparency to the existing system.
