Abraham Lincoln became a nationally known political figure based on a series of public debates with Stephen Douglas.
his victory over Douglas for a Senate seat.
his retirement from the House of Representatives.
his support of popular sovereignty.

A). A series of public debates with Stephen Douglas

Explanation:

As per the question, the key cause of Lincoln's nationwide political popularity was 'a sequence of his public debates with the former Democratic senator, Stephen A. Douglas.' The series of seven debates with him regarding the concern over extension of slavery as Douglas had the belief that slavery will automatically come to an end as the product of sovereignty while Lincoln was in favor of a proper law for the abolishment of slavery to outcast it legally. However, the debates could not reach a conclusion yet this point of view made Lincoln extremely popular among the masses.

union blockade

explanation:

the union blockade in the american civil war was a naval strategy by the united states to prevent the confederacy from trading.

the blockade was proclaimed by president abraham lincoln in april 1861, and required the monitoring of 3,500 miles (5,600 km) of atlantic and gulf coastline, including 12 major ports, notably new orleans and mobile. those blockade runners fast enough to evade the union navy could only carry a small fraction of the supplies needed. they were operated largely by british citizens, making use of neutral ports such as havana, nassau and bermuda. the union commissioned around 500 ships, which destroyed or captured about 1,500 blockade runners over the course of the war.

c) legal battles last five weeks after the election due to the narrow victory in several states.