What forced Republicans to start a new political party around the year 1848?

Answer 1

It was indeed "the collapse of the Whigs", though that doesn't explain things... better, "Kansas-Nebraska".

First, it was not quite 1848 -- the Whigs, running Zachary Taylor, won the Presidency that year. True, the party had grown rather weak by 1850, but the real "collapse" came in 1854, as a result of the Kansas-Nebraska Act, which undid the "Missouri Compromise" and allowed for the people of any territory to make their own decision about whether or not to allow slavery. (This was the "popular sovereignty" position pushed by leading Northern Democrat, Stephen Douglas, U.S. Senator from Illinois.)

Southern Whigs broke from the Northern wing of the party to support the bill along with the Democrats (both Southern and Northern, though this also caused some NORTHERN Democrats to break off and join various "Free Soil" groups, including the new Republican Party).

Not to quibble too much, but since the "Republican Party" was not already in existence it was mainly the Northern WHIGS who were "forced to form a new party", that is, if they still had some hope of wielding national power now that their Southern compatriots had largely abandoned them.

Also, the Republican Party was not the ONLY option for the Northern Whigs and other "anti-Nebraska" people. For a time it looked like many might joint the nativist "American Party" (popularly called the "Know Nothings"), but it too lost Southern supporters, and many Whigs were not so keen on their nativist (anti-immigrant) emphasis.

So, essentially, the Republicans were a combination of old Whigs, with their support of American business (pro-tariff, pro-internal improvement), and "Free Soilers", including a number of former Northern Democrats.

Answer 2

the collapse of the whig party