The Federalist Party in the United States was a significant political force during the early years of the United States' history. Wikipedia doesn't really cite reasons why the party declined and eventually ceased to exist - what would you cite as the one main reason for their decline and collapse?
A major factor was the war of 1812. The Federalist party for a long time supported peace with Britain and war with France whereas the Democratic-Republican party had the exact opposite position. Eventually, British behavor towards the US during its war with France caused anti-British and thus anti-Federalist sentiment. Quoting the Wikipedia regarding the first point:
Britain used her navy to prevent American ships from trading with France (with which Britain was at war). The United States, which was a neutral nation, considered this act to be against international law. Britain also armed Indian tribes in the Northwest Territory and encouraged them to attack settlers, even though Britain had ceded this territory to the United States by treaties in 1783 and 1794. Most insulting though was the impressment of seamen as the Royal Navy boarded American ships on the high seas...
...An angry public elected a “war hawk” Congress, led by such luminaries as Henry Clay and John C. Calhoun.
The war started with territorial ambitions on the American side but ended in status quo ante bellum. Nevertheless it was deemed as a great success, a "second war of independence" and a great wave of patriotism surrounded American victories. In particular, General Andrew Jackson became a national hero which helped him becoming a president in 1829.
Especially important in the collapse of the Fedaralist party was the Hartford Convention of 1814-1815. This mostly Federalist convention in New England ended up suggesting some constitutional amendments but was alleged to consider the secession of New England from the Union and a separate peace with Britain. After the war the convention was widely viewed as an act of treason and a black mark on the reputation of the Federalist party.
An under-appreciated reason for the collapse of the Federalists is that they were, essentially, a neo-mercantalist party. Hamilton and others were pro-industrialization not so much because they wanted to see individuals get rich through manufacturing, but because industrialization made the United States a more powerful nation in the international system (Source: Licht). Yes, we tend to think of the Federalists as the "conservative" party, and yes, Hamilton was pro-bank and pro-business, but he was foremost a state builder. In this sense, the Federalists (and later the Whigs) are better thought of as ancestors to New Deal Democrats than to 20th Century Republicans. Like FDR, the Federalists and Whigs believed in a positive role for the state.
What does this have to do with the Federalist Party's collapse? The War of 1812 disrupted manufacturing imports to the United States, which led even the Jeffersonian Republican Party to relax its policies toward industrialization. Henceforward, up-and-coming manufacturers found the laissez-faire Republican Party to be a more congenial home than the business-in-service-to-the-state Federalists. A surprising number of industrialists in the 1810s and 1820s were Republicans, not Federalists. Deprived of the industrialists who should have been the party's power base, the Federalists in the 1820s began to look increasingly hollow. (Source: Shankman)
The Republican Party in the 1820s was the classically liberal party (source: McCoy), while the Federalists were the old-fashioned mercantalists. Accordingly, as liberalism became more popular and as mercantalism became passe, the most significant political division in the country became that between different camps of liberals, the Jacksonians and Adamsites. The Jacksonians upheld the values of "negative liberty," while the Adamsites upheld the values of "positive liberty"--but both factions were essentially liberal (Source: Benson). The Federalists, by contrast, believed in order through hierarchy (Source: Appleby).
As organized mercantalism faded into irrelevancy, rump Federalists in the 1820s either retired from public life or aligned themselves with one of the new Republican factions. These factions eventually formalized the split of the Republican Party into the Democrats and the National Republicans/Whigs (Source: McCormick).
The Federalist party had been on a downslope after its initial success. This was due to the fact that its first candidate was George Washington, war hero and Virginian that was equally acceptable to the South as to the North.
The party's next standard-bearer, John Adams, barely beat the Democratic-Republicans Jefferson in 1796, and it was all downhill from there. Adams won five New England states, plus four of five "Midatlantic" states---New York, New Jersey, Delaware and Maryland, but still prevailed only with swing votes from the Jeffersonian states of Pennsylvania, Virginia, and North Carolina. In the 1800 re-match, Jefferson won Maryland and New York, putting an end to Federalist presidencies.
In the 1804 and 1808 campaigns, the Federalists won only New England states, in the latter (better) case, only the coastal states of Massachusetts, New Hampshire, Connecticut and Rhode Island. In 1812, they seemed to enjoy a revival, winning New York, New Jersey, and Delaware, but only because the former governor of New York, DeWitt Clinton, headed the ticket.
The final blow to the Federalists came during the War of 1812 when the four coastal New England states (the party's base) was suspected of not supporting the war effort, and wishing to sign a separate peace with Britain.
The Federalist party had the perception of favoring the upper class, and as a result they began to lose support of the general population. The Democratic and Republican parties started focusing on issues that appealed more to the "common man", and as a result began to sway voters away from the Federalist party until it finally ceased to exist.
People also forget, the death of Hamilton. With their leader Alexander hamilton killed by Burr. The Federalist party is left without direction or a party leader. Had Alexander been around they would have had a sense of direction and focused on the industry and common man. Since Alexander himself was an orphan, he could easily understand the needs of the common people having been one of them. We would have been better off had he won the duel. And im sure had he been alive maybe even run as president or find a succesor to run and win for his party. Of course Burr burried the Federalist.