McCormick was convinced her husband had lost the primary due to the lack of engagement of Republican women voters.
She turned her attention towards organizing Republican women, starting Republican Women's Clubs in 90 of Illinois's 102 counties.
McCormick used that newly mobilized voting base when, in 1928, she ran in a heavily contested primary race for one of Illinois's at-large congressional seats.
In the April 1928 primary election, she finished in first place in a field of eight candidates, including two sitting Congressmen, to win one of the two Republican nominations.
In November 1928, McCormick won first place in the general election with 1.7 million votes, elected along with the incumbent.
Her vote total was a larger vote share than any other Republican on the ticket in Illinois, including President Herbert Hoover, and larger than any other member of the House of Representatives that year.
McCormick was one of eight women elected to serve in the Seventy-first Congress, and one of three women elected for the first time.
By the time she entered Congress, McCormick had built a reputation as an astute politician for her years of working with her husband, and her ability to navigate the factions of Illinois politics.

Once in Congress, McCormick was appointed to the House Committee on Naval Affairs.
She was the first woman to serve on the influential committee.
Although she was not placed on the Agriculture Committee, despite her knowledge of farming, she pushed legislation to relieve farm overproduction.
McCormick worked to ensure Oscar DePriest, elected to represent First Congressional District of Illinois, was seated over the objections of southerners who wanted to block the seating of an African-American.
She supported a proposed amendment to the Census and Reapportionment Bill which aimed to enforce the 14th Amendment by counting disenfranchised individuals of voting age in the census.
She also aided constituents who were veterans of the Spanish–American War who were having difficulty with their pensions.

In September 1929, McCormick announced her intention to run for the Senate against Republican incumbent Charles S. Deneen, who had won the seat from her husband in 1924.
She sought the nomination at a time when no woman had ever been elected to the Senate.
By October, McCormick had returned to Illinois, visiting the state's various counties to rally support while Deneen was stuck in Washington, D.C., on Senate business.
As an Illinois farm owner, McCormick drew support from farmers in the state, particularly those down-state.
McCormick also campaigned on her opposition to the World Court.
She defeated Deneen in the 1930 Republican primary, 52% to 30%, to become the first female major party nominee for the Senate.
The victory showed strong support for McCormick throughout the state, including a surprisingly strong showing in Chicago where she had gained the support of Mayor William Hale Thompson, who also had a rivalry with Deneen.
McCormick later testified that the campaign cost $252,572 of her own money, with additional funds being raised from relatives.

For the general election, McCormick was up against Democratic nominee former Senator James Hamilton Lewis.
Lewis previously lost the seat to Medill in 1918.
William Thompson, who had supported McCormick in the primary, threw his support to Lewis in the general election.
One contentious issue in the campaign was Prohibition, which McCormick supported and Lewis did not.
The high cost of McCormick's primary campaign also became a point for attack in the general election, with Lewis accusing McCormick of trying to buy the election.
Lewis also made a point not to refer to McCormick by name, instead calling her "the lady candidate".
McCormick refused to make her gender an issue, calling gender differences a personality issue and insisting political party mattered more in the general election.
Unfortunately for McCormick, 1930 was a difficult year for Republican candidates as the stock market crash had occurred the year before.
McCormick lost the election, 64% to 31%.
McCormick was convinced her husband had lost the primary due to the lack of engagement of Republican women voters. She turned her attention towards organizing Republican women, starting Republican Women's Clubs in 90 of Illinois's 102 counties. McCormick used that newly mobilized voting base when, in 1928, she ran in a heavily contested primary race for one of Illinois's at-large congressional seats. In the April 1928 primary election, she finished in first place in a field of eight candidates, including two sitting Congressmen, to win one of the two Republican nominations. In November 1928, McCormick won first place in the general election with 1.7 million votes, elected along with the incumbent. Her vote total was a larger vote share than any other Republican on the ticket in Illinois, including President Herbert Hoover, and larger than any other member of the House of Representatives that year. McCormick was one of eight women elected to serve in the Seventy-first Congress, and one of three women elected for the first time. By the time she entered Congress, McCormick had built a reputation as an astute politician for her years of working with her husband, and her ability to navigate the factions of Illinois politics.
Once in Congress, McCormick was appointed to the House Committee on Naval Affairs. She was the first woman to serve on the influential committee. Although she was not placed on the Agriculture Committee, despite her knowledge of farming, she pushed legislation to relieve farm overproduction. McCormick worked to ensure Oscar DePriest, elected to represent First Congressional District of Illinois, was seated over the objections of southerners who wanted to block the seating of an African-American. She supported a proposed amendment to the Census and Reapportionment Bill which aimed to enforce the 14th Amendment by counting disenfranchised individuals of voting age in the census. She also aided constituents who were veterans of the Spanish–American War who were having difficulty with their pensions.
In September 1929, McCormick announced her intention to run for the Senate against Republican incumbent Charles S. Deneen, who had won the seat from her husband in 1924. She sought the nomination at a time when no woman had ever been elected to the Senate. By October, McCormick had returned to Illinois, visiting the state's various counties to rally support while Deneen was stuck in Washington, D.C., on Senate business. As an Illinois farm owner, McCormick drew support from farmers in the state, particularly those down-state. McCormick also campaigned on her opposition to the World Court. She defeated Deneen in the 1930 Republican primary, 52% to 30%, to become the first female major party nominee for the Senate. The victory showed strong support for McCormick throughout the state, including a surprisingly strong showing in Chicago where she had gained the support of Mayor William Hale Thompson, who also had a rivalry with Deneen. McCormick later testified that the campaign cost $252,572 of her own money, with additional funds being raised from relatives.
For the general election, McCormick was up against Democratic nominee former Senator James Hamilton Lewis. Lewis previously lost the seat to Medill in 1918. William Thompson, who had supported McCormick in the primary, threw his support to Lewis in the general election. One contentious issue in the campaign was Prohibition, which McCormick supported and Lewis did not. The high cost of McCormick's primary campaign also became a point for attack in the general election, with Lewis accusing McCormick of trying to buy the election. Lewis also made a point not to refer to McCormick by name, instead calling her "the lady candidate". McCormick refused to make her gender an issue, calling gender differences a personality issue and insisting political party mattered more in the general election. Unfortunately for McCormick, 1930 was a difficult year for Republican candidates as the stock market crash had occurred the year before. McCormick lost the election, 64% to 31%. 
Ruth Hanna McCormick