Short answers, pulled from the story.
Kennedy formally announced his candidacy on the 2nd of January 1960, at the Russell Senate Office Building in Washington, D.C. He stated he would compete in multiple primaries, including New Hampshire, and would refuse the vice-presidential nomination even if he lost the presidential race. He set up campaign headquarters at 260 Tremont Street in Boston and named his brother Robert F. Kennedy as campaign manager.
Kennedy defeated Nixon by 303 electoral votes to 219, but his national popular vote margin was just 49.7 percent to 49.5 percent. His winning margins in two contested states were 8,858 votes in Illinois and 46,266 votes in Texas. Hawaii was decided by a recount that gave Kennedy a margin of only 115 votes.
Kennedy confronted the religion issue directly before the Greater Houston Ministerial Association in September 1960. He told the assembled Protestant clergy he was not the Catholic candidate for president but the Democratic Party's candidate who also happened to be a Catholic, and pledged that his Church would not speak for him on public matters. He promised to respect the separation of church and state and to resist any Catholic official seeking to dictate public policy.
Just weeks before the election, King was arrested in Atlanta for a sit-in and sentenced to four months of hard labor. Kennedy phoned Coretta Scott King to express concern, and Robert Kennedy contacted the presiding judge, securing King's release. Martin Luther King Sr., who had backed Nixon, then publicly endorsed Kennedy. Black voter support for the Democratic ticket rose from 61 percent in 1956 to 68 percent in 1960, a shift that proved pivotal in Illinois, Michigan, and South Carolina.
Kennedy selected Johnson to balance the Democratic ticket geographically, creating what some called the "Boston-Austin axis," and because he could not be elected without the support of traditional Southern Democrats who had backed Johnson. According to biographer Robert Caro, Kennedy's calculations also included securing Texas's electoral votes and removing Johnson from his position as Senate Majority Leader so the more liberal Mike Mansfield could take the role. AFL-CIO president George Meany publicly opposed the choice, calling Johnson "the arch foe of labor."
An estimated 70 million Americans, roughly two-thirds of the electorate, watched the first debate on September 26. Gallup polls in October showed Kennedy moving into a consistent lead of 49 percent to Nixon's 46 percent, after the two had been in a statistical tie through most of August and September. Pollster Elmo Roper estimated the debates delivered Kennedy approximately two million additional votes, with most of the gain coming from the first broadcast.