Ploug was a member of the Danish Constituent Assembly in 1848, where he belonged to Orla Lehmann and Henrik Nicolai Clausen's branch, which is usually referred to as the "left wing" of the National Liberal party.

He gave the war of 1864 his warmest support, and he encouraged the people to fight, partly in the blind belief that Sweden would come to Denmark's military aid.

[2][3] As a young man, Ploug was strongly committed to personal freedom and advocated universal suffrage (for men), but over the years he went in a more conservative direction.

[6] He exploited these talents, for example, in a speech to the Swedish Scandinavists in Kalmar in 1843, where he was greeted with cheers when he claimed that Southern Jutland was in danger of being conquered by "Germanness".

[10] The plan was well received by the two royal houses, but the mutual assurances of brotherhood remained rhetorical and suffered final defeat when Sweden-Norway failed to support Denmark militarily during the Second Schleswig War.

[11] Ploug's uncompromising approach meant that even in the "golden age" of national liberalism from 1855 to 1863, he found himself at odds with leading politicians.

[13] He also sought, when defeat was a fact, to revive the Scandinavian idea of union, but the Swedish "allies" did not want to be involved in a struggle for the reconquest of Schleswig.

[14] After the defeat, the National Liberal leaders moved to the right in Danish politics, and Ploug was elected to several positions of trust in the party Højre.

[4] He succeeded, along with Carl Christian Hall, to influence Højre in a romantic nationalist direction, although the party maintained its support for the king.

He was active at student gatherings and at the folk festivals on Skamlingsbanken, for which he also wrote a number of songs playing on national sentiment.

[4] In 1842 he wrote "Længe var Nordens herlige Stamme", which in the following years became a kind of battle song for Scandinavianism.

Impressed by the Danish evacuation of Danevirke in 1848, he wrote "Den danske sang: Liflig fløjte Velsklands Nattergale".

[11] In Ploug's defence, it has been emphasized that he never took personal advantage of his views, just as his contributions to Fædrelandet took no account of friend or foe.