He sought to position himself for contention in the 1848 presidential election, but his vote to lower the tariff destroyed his base of support in his home state.

He was educated privately at Quaker-run preparatory schools, before studying at the College of New Jersey (now Princeton University), from which he graduated with highest honors in 1810.

[2] As a new graduate, Dallas had little enthusiasm for legal practice; he wanted to fight in the War of 1812, a plan that he dropped due to his father's objection.

[2] Dallas enjoyed the opportunities offered to him by being in Russia, but after six months there he was ordered to go to London to determine whether the War of 1812 could be resolved diplomatically.

[2] There, he was appointed by James Madison to become the remitter of the treasury, which is considered a "convenient arrangement" because Dallas' father was serving at the time as that department's secretary.

[2] Since the job did not entail a large workload, Dallas found time to develop his grasp of politics, his major vocational interest.

[2] However, he quickly grew bored with that post, and became the United States Attorney for the eastern district of Pennsylvania in 1829, a position his father had held from 1801 to 1814, and continued in that role until 1831.

[2] In December of that year, he won a five-man, eleven-ballot contest in the state legislature for Senator from Pennsylvania, to complete the unexpired term of Isaac D. Barnard, who had resigned.

[12] He was appointed by President Martin Van Buren as Envoy Extraordinary and Minister Plenipotentiary to Russia in 1837, serving until 1839, when he was recalled at his own request.

[9] In the lead-up to the 1844 presidential election, Dallas supported Van Buren for the Democratic nomination over fellow Pennsylvanian James Buchanan.

However, Dallas' reluctant vote to lower a tariff destroyed much of his base in Pennsylvania, and his advocacy of popular sovereignty on the question of slavery strengthened opposition against him.

At the end of his vice-presidential term, Dallas said he had cast thirty tie-breaking votes during his four years in office (although only nineteen of these have been identified in Senate records).

Taking obvious personal satisfaction in this record, Dallas singled out this achievement and the fairness with which he believed he accomplished it in his farewell address to the Senate.

(A twenty-eighth vote in favor was held in reserve by a senator who opposed the measure but agreed to follow the instructions of his state legislature to support it.)

When he cast the tie-breaking vote in favor of the tariff on July 28, 1846, Dallas rationalized that he had studied the distribution of Senate support and concluded that backing for the measure came from all regions of the country.

While his action, based on a mixture of party loyalty and political opportunism, earned Dallas the respect of the president and certain party leaders—and possible votes in 1848 from the southern and western states that supported low tariffs—it effectively demolished his home state political base, ending any serious prospects for future elective office.

(He even advised his wife in a message hand-delivered by the Senate Sergeant at Arms, "If there be the slightest indication of a disposition to riot in the city of Philadelphia, owing to the passage of the Tariff Bill, pack up and bring the whole brood to Washington.")

While Dallas' tariff vote destroyed him in Pennsylvania, his aggressive views on Oregon and the Mexican War crippled his campaign efforts elsewhere in the nation.

In his last hope of building the necessary national support to gain the White House, the Vice President shifted his attention to the aggressive, expansionist foreign policy program embodied in the concept of "Manifest Destiny".

At the very beginning of his diplomatic service in Britain, he was called to act upon the Central American question and the US request that Sir John Crampton, the British minister to the United States, should be recalled.