During the same year in the 1876 United States presidential election, he joined the "Conkling for President" movement when he attended a Republican National Convention for the first time.

His election was ensured due to an intraparty compromise with Republican Half-Breed candidate Chauncey M. Depew, who also sought the seat.

However, he served only from March 4 to May 16, 1881, when he and Conkling resigned because of a disagreement with President James Garfield over federal appointments in New York.

Soon afterward, however, Garfield's assassination by Charles J. Guiteau, a self-proclaimed Stalwart who claimed friendships with Platt and Conkling, was the finishing blow for their faction.

Garfield's nomination of Robertson forced the junior senator to remain loyal to his vows and buck Conkling, or affirm his Stalwart politics and break the promise.

A cartoon published by Thomas Nast in Harper's Weekly portrayed the pair as having "lost their heads," with former president Ulysses S. Grant struggling to resurrect Conkling.

[11] The reasoning for the resignations, Platt articulated, functioned as a "desperate remedy" to prevent him from being forced to vote on confirming Robertson to the collector position.

[11] A majority in the chambers viewed that Garfield's promotion of Robertson at the opposition of the "regular" machine were not "dishonest" nor "dishonorable."

Allies of Conkling bitterly fought to reinstate them to their seats, including Vice President Chester Arthur, although Platt would withdraw his name from the special election following a month of balloting.

[11] The assassination of Garfield resulted in subsequent suspicions toward Stalwarts, and an alliance between Half-Breed and Independents thwarted the "Old Guard" by nominating Warner Miller and Elbridge G. Lapham to succeed Platt and Conkling respectively.

The inability of Platt to ensure a successful strategy that only resulted in the self-ousting of the senatorial pair ended his friendship with Conkling.

"[4] During the 1884 United States presidential election, the Republican convention nominated Half-Breed leader James G. Blaine to lead the party ticket.

This resulted in an intraparty schism, with a "Mugwump" faction refusing to support Blaine and instead backing Bourbon Democrat Grover Cleveland.

Although Platt had allied with the Stalwarts which fiercely opposed Blaine (who long had a personal rivalry with Conkling), he supported the nominee for the general election.

[9] Blaine ultimately lost the race to Cleveland, which in part was attributed to the influence of Harper's Weekly Mugwump cartoonist Thomas Nast.

Two years after his first wife died in 1901, he married Lillian Janeway, whom the New York Times described as "young enough in appearance to pass for his daughter.

"[17] Their legal separation was announced in 1906, with Platt agreeing to pay his estranged wife $75,000 in exchange for her dropping all financial claims upon him and dismissing a suit for divorce which had been previously filed.

[7] He retired from the Senate in 1909 and was stricken by what was diagnosed as an acute attack of Bright's disease on May 28, 1909, a case so severe that his doctor publicly predicted his patient's imminent demise.

[7] Seemingly restored to health, Platt was suddenly stricken by a second attack of kidney disease at about 1 pm on March 6, 1910.