Although Taft entered office determined to advance Roosevelt's Square Deal domestic agenda, he stumbled badly during the Payne–Aldrich Tariff Act debate and the Pinchot–Ballinger controversy.

[9] Progressive Republican leader Robert M. La Follette had already announced a challenge to Taft for the 1912 Republican nomination, but many of his supporters shifted to Roosevelt after the former president decided to seek a third presidential term, which was permissible under the Constitution prior to the ratification of the Twenty-second Amendment.

At the 1912 Republican National Convention, Taft narrowly defeated Roosevelt for the party's presidential nomination.

The party's platform built on Roosevelt's Square Deal domestic program and called for several progressive reforms.

Proposals on the platform included restrictions on campaign finance contributions, a reduction of the tariff and the establishment of a social insurance system, an eight-hour workday and women's suffrage.

Party members also had different outlooks on foreign policy, with pacifists like Jane Addams opposing Roosevelt's call for a naval build-up.

Nine of the states where progressive elements were strongest had set up preference primaries, which Roosevelt won, but Taft worked harder than Roosevelt to control the Republican Party's organizational operations and the mechanism for choosing its presidential nominee, the 1912 Republican National Convention.

[11] The leadership of the new party at the level just below Roosevelt included Jane Addams of Hull House, a leader in social work, feminism, and pacifism;[12] former Senator Albert J. Beveridge of Indiana, a leading advocate of regulating industry;[13] Gifford Pinchot, a leading environmentalist.

[16] The two main organizers were Senator Joseph M. Dixon of Montana and especially George W. Perkins, a senior partner of the Morgan bank who came from the efficiency movement.

However, Perkins' close ties to Wall Street made him deeply distrusted by many party activists.

The exception was California, where the progressive element took control of the Republican Party and Taft was not even on the November ballot.

Republican legislators, governors, national committeemen, publishers, and editors showed comparable reluctance as bolting the old party risked career suicide.

As a result, most of Roosevelt's previous political allies supported Taft, including his son-in-law, Representative Nicholas Longworth of Cincinnati.

[20] Yet he alienated white Southern supporters, Lily-white movement, on the eve of the election by publicly dining with black people at a Rhode Island hotel.

It spoke with near-religious fervor and the candidate himself promised: "Our cause is based on the eternal principle of righteousness; and even though we, who now lead may for the time fail, in the end the cause itself shall triumph".

The platform's main theme was reversing the domination of politics by business interests, which allegedly controlled the Republican and Democratic parties alike.

[30] The platform in general expressed Roosevelt's "New Nationalism", an extension of his earlier philosophy of the Square Deal.

He called for new restraints on the power of federal and state judges along with a strong executive to regulate industry, protect the working classes and carry on great national projects.

Though the platform called for limiting naval armaments, it also recommended the construction of two new battleships per year, much to the distress of outright pacifists such as Jane Addams.

On October 14, 1912, while Roosevelt was campaigning in Milwaukee, Wisconsin, he was shot by John Flammang Schrank, but the bullet lodged in his chest only after penetrating both his steel eyeglass case and a 50-page single-folded copy of the speech titled "Progressive Cause Greater Than Any Individual", he was to deliver, carried in his jacket pocket.

[36] As an experienced hunter and anatomist, Roosevelt correctly concluded that since he was not coughing blood, the bullet had not reached his lung and he declined suggestions to go to the hospital immediately.

His opening comments to the gathered crowd were: "Ladies and gentlemen, I don't know whether you fully understand that I have just been shot, but it takes more than that to kill a Bull Moose".

Doctors concluded that it would be less dangerous to leave it in place than to attempt to remove it and Roosevelt carried the bullet with him for the rest of his life.

[42] Both Taft and Democratic nominee Woodrow Wilson suspended their own campaigning until Roosevelt recovered and resumed his.

When asked if the shooting would affect his election campaign, he said to the reporter "I'm fit as a bull moose", which inspired the party's emblem.

[47] In reaction to Roosevelt's proposals for popular overrule of court decisions, Taft said, "Such extremists are not progressives—they are political emotionalists or neurotics".

Five delegates from each convention met to negotiate and the Progressives wanted reunification with Roosevelt as nominee, which the Republicans adamantly opposed.

The Progressives suggested Hughes as a compromise candidate, then Roosevelt sent a message proposing conservative senator Henry Cabot Lodge.

All the remaining Progressives in Congress rejoined the Republican Party, except Whitmell Martin, who became a Democrat.

Robert M. La Follette Sr. broke bitterly with Roosevelt in 1912 and ran for president on his own ticket, the 1924 Progressive Party, during the 1924 presidential election.