President Joe Biden’s abrupt decision to bow out of the presidential race and endorse Vice President Kamala Harris to be the Democratic candidate against former President Donald Trump has caused a political earthquake. It also changes the contours of a presidential race — which most voters said they did not want to see — that has seemed rigidly set for more than a year. Since Biden’s disastrous debate in June, the Democratic party has been in disarray. But on Sunday, Democrats started lining up behind Harris. Dozens of members of Congress and senators endorsed her.