Complex sentence: Leonidas L. Polk was the initial frontrunner for the presidential nomination, having been instrumental in the party's formation and holding great appeal to its agrarian base, but he unexpectedly died while in Washington D.C. on June 11.
Split into simpler sentences: Leonidas L. Polk was the initial frontrunner for the presidential nomination. He had been instrumental in the party's formation and held great appeal with its agrarian base, but he unexpectedly died while in Washington, D.C., on June 11.