The Republican National Convention in 1860 brought McClure greater political recognition. Long identified as an opponent of Simon Cameron, an important figure in the Pennsylvania Republican organization, McClure could only get a place on the state's delegation to the Republican National Convention by pledging to support his old foe Cameron. Once at the party conclave, the young editor and his allies on the delegation played key roles in winning the state's vote for Illinois's Abraham Lincoln rather than New York's William Seward.

Alexander Kelly McClure was born in Sherman’s Valley, Pennsylvania to the farming family of Alexander and Isabella Anderson McClure on January 9, 1828. He received little formal schooling and was apprenticed to a tanner in 1843. He also assisted as a printer at the local Perry County Freeman, and so began a long and distinguished career as a newspaperman.

As was the case with many late nineteenth-century public figures, the Civil War became the moral reference point for Cullom's political career. Unlike most others, he spent the war years as a Republican politician, not a soldier. Initially wary of the radical image of the new party, Cullom supported Millard Fillmore in 1856 and was elected to the state legislature on a combined Fillmore-Free Soil ticket. Then in 1858, he took the biggest (and perhaps the only) gamble of his career by hitching his fortunes to Lincoln and the Republicans.